Archive for the ‘Java’ Category
Java Gregorian Date
One of my students asked for an example of how to work with a Gregorian date and timezones in Java. I dug out an old example file from when I taught Java at Regis University.
The code follows:
/* || Program name: MyGregorian.java || Created by: Michael McLaughlin || Creation date: 10/07/02 || History: || ---------------------------------------------------------------------- || Date Author Purpose || -------- ---------------------- --------------------------------- || dd/mm/yy {Name} {Brief statement of change.} || ---------------------------------------------------------------------- || Execution method: Static class demonstrating timezone setting. || Program purpose: Designed as a stand alone program. */ // Class imports. import java.util.Calendar; import java.util.GregorianCalendar; import java.util.TimeZone; // Define MyGregorian class. public class MyGregorian { // Testing static main() method. public static void main(String args[]) { // Set an initial variable. String initial = ""; System.out.println("======================================================"); System.out.println("Value of [user.timezone]: [" + (initial = (null != System.getProperty("user.timezone")) ? "Unset" : System.getProperty("user.timezone")) + "]"); System.out.println("======================================================"); GregorianCalendar gc = (GregorianCalendar) Calendar.getInstance(); System.out.println("Calendar Date: [" + gc.getTime() + "]"); gc.add(GregorianCalendar.MONTH,1); System.out.println("Calendar Date: [" + gc.getTime() + "]"); System.out.println("======================================================"); System.out.println("Value of [user.timezone]: [" + System.getProperty("user.timezone") + "]"); System.out.println("Value of [user.timezone]: [" + System.setProperty("user.timezone","") + "]"); gc.add(GregorianCalendar.MONTH,1); System.out.println("Calendar Date: [" + gc.getTime() + "]"); System.out.println("======================================================"); Calendar c = Calendar.getInstance(); System.out.println("Calendar Date: [" + c.getTime() + "]"); // Move the date ahead one month, hour and minute. c.add(Calendar.MONTH,1); System.out.println("Calendar Date: [" + c.getTime() + "]"); System.out.println("======================================================"); } // End of testing static main() method. } // End of MyGregorian class. |
It prints to console:
====================================================== Value of [user.timezone]: [Unset] ====================================================== Calendar Date: [Thu May 05 23:43:42 MDT 2022] Calendar Date: [Sun Jun 05 23:43:42 MDT 2022] ====================================================== Value of [user.timezone]: [America/Denver] Value of [user.timezone]: [America/Denver] Calendar Date: [Tue Jul 05 23:43:42 MDT 2022] ====================================================== Calendar Date: [Thu May 05 23:43:42 MDT 2022] Calendar Date: [Sun Jun 05 23:43:42 MDT 2022] ====================================================== |
As always, I hope this helps those who need to see and example to work with Gregorian dates.
PL/SQL Inheritance Failure
PL/SQL is a great programming language as far as it goes but it lacks true type inheritance for its collections. While you can create an object type and subtype, you can’t work with collections of those types the same way. PL/SQL object type inheritance, unlike the Java class hierarchy and parallel array class hierarchy, only supports a class hierarchy. Effectively, that means:
- You can pass a subtype as a call parameter, or argument, to a parent data type in a function, procedure, or method signature, but
- You can’t pass a collection of a subtype as a call parameter, or argument, to a collection of parent type in a function, procedure, or method signature.
The limitation occurs because collections have their own data type, which is fixed when you create them. Worse yet, because Oracle has never seen fit to fix their two underlying code trees (23 years and counting since Oracle 8i), you have two types of collections using two distinct C/C++ libraries. You define collections of Attribute Data Types (ATDs) when you create a collection of a standard scalar data type, like NUMBER, VARCHAR2, or DATE. You define collection of User-Defined Data Types (UTDs) when you create a collection of a SQL UDT or PL/SQL-only RECORD data type. The former uses one C/C++ library and the latter another.
Now, Oracle even make the differences between Java and PL/SQL more complex because it treats collections known as tables, really lists in most programming languages, differently than varrays, or arrays. You create a TABLE collection, or list, when you create a table of a scalar or UDT data type. There are two options when you create these object types, and they are:
- You create an empty collection with a no element constructor, which means you’ll need to allocate memory before assigning element values later in your program.
- You create a populated collection with a comma-delimited list of elements.
Both approaches give you a list of elements with a densely populated index. A “densely populated index” is Oracle’s jargon for how they characterize a 1-based sequence of integers without any gaps (e.g., 1, 2, 3, …). The initial construction works the same way whether you create a TABLE or VARRAY collection type. Unfortunately, after you’ve built the collection behaviors change. If you use Oracle’s Collection API to delete one or more items from a TABLE collection type, you create gaps in the index’s sequence of values. That means you must use special logic to navigate across a TABLE collection type to ensure it doesn’t fail when encountering a gap in the numeric sequence.
For example, here’s a FOR-LOOP without the logic to vouchsafe a uninterrupted set of sequence values incrementing by a counter of 1 element at a time:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | DECLARE /* Create a local table collection. */ TYPE list IS TABLE OF VARCHAR2(10); /* Statically allocate memory and assign values to for elements. */ lv_list LIST := list('Moe','Shemp','Larry','Curly'); BEGIN /* Remove the second element, Shemp, from the collection of variable length strings. */ lv_list.DELETE(2); /* Loop through the target with a for loop, which depends on densely populated index values. */ FOR i IN 1..lv_list.COUNT LOOP dbms_output.put_line('['||lv_list(i)||']'); END LOOP; END; / |
The program fails when it tries to read the second element of the table collection, which was previously removed. It raises the following error message after print the first element of the table collection:
[Moe] DECLARE * ERROR at line 1: ORA-01403: no data found ORA-06512: at line 16 |
Conveniently, Oracle’s Collection API provides an EXISTS method that we can use to check for the presence of an index’s value. Modifying line 16 by wrapping it in an IF-statement fixes one problem but identifies another:
15 16 17 18 19 | FOR i IN 1..lv_list.COUNT LOOP IF lv_list.EXISTS(i) THEN dbms_output.put_line('['||lv_list(i)||']'); END IF; END LOOP; |
The program no longer fails on a missing index value, or index gap, but it returns fewer lines of output than you might expect.
That’s because the Oracle Collection API’s COUNT method returns the number of elements currently allocated in memory not the number of original elements. We learn that when we deleted the second element, Oracle deleted the memory allocated for it as well. This is the type of behavior you might expect for a singly linked list. It prints:
[Moe] [Larry] |
One more change is required to count past and to the highest index value. One line 15, change the COUNT method call to the LAST method call, which returns the highest index value.
15 16 17 18 19 | FOR i IN 1..lv_list.LAST LOOP IF lv_list.EXISTS(i) THEN dbms_output.put_line('['||lv_list(i)||']'); END IF; END LOOP; |
It now prints the three stooges we would expect to see:
[Moe] [Larry] [Curly] |
Realistically, a FOR-LOOP is not the best control structure for a collection. You should use a WHILE-LOOP and treat the incrementing value as an iterator rather than sequence index value. An iterator doesn’t worry about gaps in the sequence, it simply moves to the next element in the singly linked list. Here’s an example that uses the iterator approach with a WHILE-LOOP:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | DECLARE /* Create a local table collection. */ TYPE list IS TABLE OF VARCHAR2(10); /* Statically allocate memory and assign values to for elements. */ lv_list LIST := list('Moe','Shemp','Larry','Curly'); /* Declare a current index variable. */ CURRENT NUMBER; BEGIN /* Remove the second element, Shemp, from the collection of variable length strings. */ lv_list.DELETE(2); /* Loop through the target with a while loop, which doesn't depend on densely populated index values by setting the starting index value and increment as if with an iterator. */ CURRENT := lv_list.FIRST; WHILE NOT (CURRENT > lv_list.LAST) LOOP dbms_output.put_line('['||lv_list(CURRENT)||']'); CURRENT := lv_list.NEXT(CURRENT); END LOOP; END; / |
The iterator approach prints the elements as:
[Moe] [Larry] [Curly] |
You can reverse the process with the following changes to lines 20-24:
20 21 22 23 24 | CURRENT := lv_list.LAST; WHILE NOT (CURRENT < lv_list.FIRST) LOOP dbms_output.put_line('['||lv_list(CURRENT)||']'); CURRENT := lv_list.PRIOR(CURRENT); END LOOP; |
It prints the list backwards:
[Curly] [Larry] [Moe] |
After covering the issues with sparsely populated, those with gaps in the sequence of indexes values, table collections, let’s examine how you must work around PL/SQL’s lack of a parallel array class hierarchy. The solution lies in combining two programming concepts:
- A function to pack the sparsely populated table collection into a densely populated one, and
- A package with overloaded functions that pack different table collections.
To develop the test case, let’s use an ADT collection because it’s the simplest to work with. The following creates a table collection of a thirty character long scalar string:
1 2 3 | CREATE OR REPLACE TYPE list IS TABLE OF VARCHAR2(30); / |
The following pack function takes a table collection of the thirty character long scalar string, evaluates the string for missing elements, and packs the existing elements into a densely populated list:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | CREATE OR REPLACE FUNCTION pack ( pv_list LIST ) RETURN list IS /* Declare a new list. */ lv_new LIST := list(); BEGIN /* Read, check, and pack an old list into a new one. */ FOR i IN 1..pv_list.LAST LOOP IF pv_list.EXISTS(i) THEN lv_new.EXTEND; lv_new(lv_new.COUNT) := pv_list(i); END IF; END LOOP; RETURN lv_new; END; / |
This anonymous block tests the pack function:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | DECLARE /* Declare a list value. */ lv_test LIST := list('Moe','Shemp','Larry','Curly'); BEGIN /* Remove one element in the middle. */ lv_test.DELETE(2); /* Pack the list of elements into a sequence of values. */ lv_test := pack(lv_test); /* Print the list of elements from the packed list. */ FOR i IN 1..lv_test.COUNT LOOP dbms_output.put_line('['||lv_test(i)||']'); END LOOP; END; / |
It prints the expected three string values:
[Moe] [Larry] [Curly] |
Now, let’s expand the example to build an overloaded package. The first step requires building a base_t object type and a table collection of the object type, like:
1 2 3 4 5 6 7 8 9 | CREATE OR REPLACE TYPE base_t IS OBJECT ( oid NUMBER ) INSTANTIABLE NOT FINAL; / CREATE OR REPLACE TYPE base_list IS TABLE OF base_t; / |
Next, you create a book_t subtype of the base_t object type and a book_list table collection of the book_t subtype, like:
1 2 3 4 5 6 7 8 9 | CREATE OR REPLACE TYPE book_t UNDER base_t ( title VARCHAR2(30) , COST NUMBER); / CREATE OR REPLACE TYPE book_list IS TABLE OF book_t; / |
We can test the base_t and book_t default constructors with the following SQL*Plus formatting and SQL query:
COL oid FORMAT 999 COL title FORMAT A20 COL COST FORMAT 99.99 SELECT * FROM TABLE(book_list(book_t(1,'Neuromancer',15.30) ,book_t(2,'Count Zero',7.99) ,book_t(3,'Mona Lisa Overdrive',7.99) ,book_t(4,'Burning Chrome',8.89))); |
It prints the following output:
OID TITLE COST ---- -------------------- ------ 1 Neuromancer 15.30 2 Count Zero 7.99 3 Mona Lisa Overdrive 7.99 4 Burning Chrome 8.89 |
The following is an overloaded package specification:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | CREATE OR REPLACE PACKAGE packer IS /* A simple ADT list of strings. */ FUNCTION pack ( pv_list LIST ) RETURN list; /* A UDT list of base objects. */ FUNCTION pack ( pv_list BASE_LIST ) RETURN base_list; /* A UDT list of subtype objects. */ FUNCTION pack ( pv_list BOOK_LIST ) RETURN book_list; END; / |
After you create the package specification, you need to provide the implementation. This is typical in any programming language that supports Interface Description Language (IDL). A package body provides the implementation for the package specification. The package body follows:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 | CREATE OR REPLACE PACKAGE BODY packer IS /* A simple ADT list of strings. */ FUNCTION pack ( pv_list LIST ) RETURN list IS /* Declare a new list. */ lv_new LIST := list(); BEGIN /* Read, check, and pack an old list into a new one. */ FOR i IN 1..pv_list.LAST LOOP IF pv_list.EXISTS(i) THEN lv_new.EXTEND; lv_new(lv_new.COUNT) := pv_list(i); END IF; END LOOP; RETURN lv_new; END pack; /* A simple ADT list of strings. */ FUNCTION pack ( pv_list BASE_LIST ) RETURN base_list IS /* Declare a new list. */ lv_new BASE_LIST := base_list(); BEGIN /* Read, check, and pack an old list into a new one. */ FOR i IN 1..pv_list.LAST LOOP IF pv_list.EXISTS(i) THEN lv_new.EXTEND; lv_new(lv_new.COUNT) := pv_list(i); END IF; END LOOP; RETURN lv_new; END pack; /* A simple ADT list of strings. */ FUNCTION pack ( pv_list BOOK_LIST ) RETURN book_list IS /* Declare a new list. */ lv_new BOOK_LIST := book_list(); BEGIN /* Read, check, and pack an old list into a new one. */ FOR i IN 1..pv_list.LAST LOOP IF pv_list.EXISTS(i) THEN lv_new.EXTEND; lv_new(lv_new.COUNT) := pv_list(i); END IF; END LOOP; RETURN lv_new; END pack; END packer; / |
The test case for the base_list object type is:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | DECLARE lv_test BASE_LIST := base_list(base_t(1),base_t(2) ,base_t(3),base_t(4)); BEGIN /* Remove one element in the middle. */ lv_test.DELETE(2); /* Pack the list of elements into a sequence of values. */ lv_test := packer.pack(lv_test); /* Print the list of elements from the packed list. */ FOR i IN 1..lv_test.LAST LOOP dbms_output.put_line('['||lv_test(i).oid||']'); END LOOP; END; / |
It prints the following output:
[1] [3] [4] |
The test case for the book_list object type is:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | DECLARE lv_test BOOK_LIST := book_list(book_t(1,'Neuromancer',15.30) ,book_t(2,'Count Zero',7.99) ,book_t(3,'Mona Lisa Overdrive',7.99) ,book_t(4,'Burning Chrome',8.89)); BEGIN /* Remove one element in the middle. */ lv_test.DELETE(2); /* Pack the list of elements into a sequence of values. */ lv_test := packer.pack(lv_test); /* Print the list of elements from the packed list. */ FOR i IN 1..lv_test.LAST LOOP dbms_output.put_line( '['||lv_test(i).oid||']' ||'['||lv_test(i).title||']' ||'['||lv_test(i).COST||']'); END LOOP; END; / |
It prints the following output:
[1][Neuromancer][15.3] [3][Mona Lisa Overdrive][7.99] [4][Burning Chrome][8.89] |
In conclusion, you would not have to write overloaded methods for every list if PL/SQL supported class hierarchy and parallel array class hierarchy like Java. Unfortunately, it doesn’t and likely won’t in the future. You can pack table collections as a safety measure when they’re passed as parameters to other functions, procedures, or methods with the code above.
As always, I hope this helps those looking for a solution.
SQL Developer JDK
In my classes, we use a VMware Linux install with SQL Developer. One of my students called me in a panic after an upgrade of packages when SQL Developer failed to launch. The student was astute enough to try running it from the command line where it generates an error like:
Oracle SQL Developer Copyright (c) 2005, 2018, Oracle and/or its affiliates. All rights reserved. /opt/sqldeveloper/sqldeveloper/bin/../../ide/bin/launcher.sh: line 954: [: : integer expression expected The JDK (/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.222.b10-0.fc30.x86_64/) is not a valid JDK. The JDK was specified by a SetJavaHome directive in a .conf file or by a --setjavahome option. Type the full pathname of a JDK installation (or Ctrl-C to quit), the path will be stored in /home/student/.sqldeveloper/19.2.0/product.conf Error: Unable to get APP_JAVA_HOME input from stdin after 10 tries |
The error is simple, the SQL Developer package update wipe clean the configuration of the SetJavaHome variable in the user’s ~/.sqldeveloper/19.2.0/product.conf file. The fix is three steps because its very likely that the Java packages were also updated. Here’s how to fix it:
- Navigate to the directory where you’ve installed the Java Virtual Machine (JVM) and find the current version of the JVM installed:
cd /usr/lib/jvm ls java*
It will return a set of files, like:
java java-1.8.0 java-1.8.0-openjdk java-1.8.0-openjdk-1.8.0.252.b09-0.fc30.x86_64 java-openjdk jre jre-1.8.0 jre-1.8.0-openjdk jre-1.8.0-openjdk-1.8.0.252.b09-0.fc30.x86_64 jre-openjdk
- Navigate to your user’s product configuration file with this command:
cd ~/.sqldeveloper/19.2.0
- Add the following line to the
product.conffile:# SetJavaHome /path/jdk SetJavaHome /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.252.b09-0.fc30.x86_64/
Now, you should be able to run it from the command line. The shortcut icon should also work if one was installed. Also, don’t forget to update your $JAVA_HOME variable in the master Bash resource file, or your local user’s .bashrc files.
As always, I hope this helps those looking for a quick solution.
MySQL 8.0.20 JDBC
In April, when I updated from MySQL 8.0.17 to MySQL 8.0.19, I found that my Java connection example failed. That’s because of a change in the JDBC driver, which I blogged about then. Starting yesterday, I began updating a base Fedora 30 configuration again to MySQL 8.0.20. I wrote a testing program for the Java JDBC file last time, and when I ran it this time it told me that I didn’t have the JDBC driver installed, or in the $CLASSPATH. My Java diagnostic script, MySQLDriver.java, returned the following error message:
Error: Could not find or load main class MySQLDriver |
The Java JDBC test program code is in the prior post. It simply loads the user, password, database, host, and port statically for my student image, which you can update for your MySQL deployment.
You can find the MySQL 8.0 Connector/J installation in Chapter 4 of the MySQL Connector/J 8.0 Developer Guide. The link to page where you can download the Connector/J driver is in Chapter 4, but to save time you can click on the following download link. You’ll find Ubuntu, Debian, SUSE Enterprise Linux Server, Red Hat Enterprise Linux/Oracle Linux, Fedora, Platform Independent, and Source Code at that download link.
I downloaded Fedora 30, 31, and 32 because eventually they get deprecated and sometimes I need the deprecated versions. Since Python 2.7 is deprecated, keeping an old Fedora 30 with both Python 2.7 and 3.7 is very attractive as a developer, keeping these Java libraries is also a potential convenience. On the Fedora 30 platform, you should download:
mysql-connector-java-8.0.20-1.fc30.noarch.rpm |
You install the RPM package with the following syntax:
rpm -ivh mysql-connector-java-8.0.20-1.fc30.noarch.rpm |
It installs a single mysql-connector-java.jar file in the /usr/share/java directory. Then, you need to export the Java Archive file like so in your .bashrc file:
# Set the Java CLASSPATH path. export CLASSPATH=/usr/share/java/mysql-connector-java.jar:. |
Now, my MySQLDriver Java program returned:
Cannot connect to database server: The server time zone value 'MDT' is unrecognized or represents more than one time zone. You must configure either the server or JDBC driver (via the 'serverTimezone' configuration property) to use a more specifc time zone value if you want to utilize time zone support. |
As qualified in the prior post, it required me to add the following line to my /etc/my.cnf configuration file, which synchronizes the database’s timezone with the operating system.
# Synchronize the MySQL clock with the computer system clock. default-time-zone='+00:00' |
You need to restart the MySQL Daemon to apply the change, like:
sudo systemctl restart mysqld.service |
Then, running my MySQL Driver connection test program worked like a charm. It returns the following:
Database connection established MySQLDriver Version [8.0.20] Database connection terminated |
As always, I hope this helps those looking for a quick solution.
SQL Developer Error
It’s been a couple releases trying to fix the following error thrown by SQL Developer on Fedora 30 and shown as the following dialog:
When you click the Detail button it shows the following error stack:
java.lang.NoClassDefFoundError: javafx/embed/swing/JFXPanel at oracle.dbtools.raptor.javafx.ui.JFXPanelFactory.createJFXPanelImpl(JFXPanelFactory.java:58) at oracle.dbtools.raptor.javafx.ui.JFXPanelFactory.createJFXPanel(JFXPanelFactory.java:34) at oracle.dbtools.raptor.startpage.StartPageViewer.createGUIComponent(StartPageViewer.java:179) at oracle.dbtools.raptor.startpage.StartPageViewer.getEditorContent(StartPageViewer.java:136) at oracle.ide.editor.AsynchronousEditor$2.run(AsynchronousEditor.java:345) at oracle.ide.editor.AsynchronousEditor$5.run(AsynchronousEditor.java:555) at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1443) at org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:68) at org.openide.util.lookup.Lookups.executeWith(Lookups.java:303) at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:2058) Caused by: java.lang.ClassNotFoundException: javafx.embed.swing.JFXPanel cannot be found by oracle.sqldeveloper_19.2.0 at org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:501) at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:421) at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:412) at org.eclipse.osgi.internal.baseadaptor.DefaultClassLoader.loadClass(DefaultClassLoader.java:107) at org.netbeans.modules.netbinox.NetbinoxLoader.loadClass(NetbinoxLoader.java:81) at java.lang.ClassLoader.loadClass(ClassLoader.java:352) ... 10 more |
I thought applying the Open Java FX package might fix the problem. I installed the package like the following:
yum install -y openjfx |
The installation log:
Last metadata expiration check: 4:03:29 ago on Tue 21 Apr 2020 06:42:26 PM MDT. Dependencies resolved. ============================================================================================= Package Architecture Version Repository Size ============================================================================================= Installing: openjfx x86_64 8.0.202-8.b07.fc30 updates 8.8 M Transaction Summary ============================================================================================= Install 1 Package Total download size: 8.8 M Installed size: 11 M Downloading Packages: openjfx-8.0.202-8.b07.fc30.x86_64.rpm 2.5 MB/s | 8.8 MB 00:03 --------------------------------------------------------------------------------------------- Total 2.1 MB/s | 8.8 MB 00:04 Running transaction check Transaction check succeeded. Running transaction test Transaction test succeeded. Running transaction Preparing : 1/1 Installing : openjfx-8.0.202-8.b07.fc30.x86_64 1/1 Running scriptlet: openjfx-8.0.202-8.b07.fc30.x86_64 1/1 Verifying : openjfx-8.0.202-8.b07.fc30.x86_64 1/1 Installed: openjfx-8.0.202-8.b07.fc30.x86_64 Complete! |
After installing the software, I determined the new JAR files. Then, I added them to my $CLASSPATH environment variable, like:
export CLASSPATH=/usr/share/java/mysql-connector-java.jar:/usr/lib/jvm/openjfx/rt/lib/ext/fxrt.jar:/usr/lib/jvm/openjfx/rt/lib/jfxswt.jar:. |
While it appears to load faster with these JAR files, it still raises the same Dialog error. I simply have to continue to look for a complete fix.
Java & MySQL 8.0.19
It’s the in-between term time and we’re all stuck at home. I decided to update the image for my Fedora 30 virtual machine. I had a work around to the update issue that I had encountered last October in Bug #96969 but it was not required with the current version. However, after updating from MySQL 8.0.17 to MySQL 8.0.19, I found that my Java connection example failed.
The $CLASSPATH value was correct:
/usr/share/java/mysql-connector-java.jar:. |
The first error that I got was the my reference to MySQL JDBC driver was incorrect. The error message is quite clear:
Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary. Cannot connect to database server: The server time zone value 'MDT' is unrecognized or represents more than one time zone. You must configure either the server or JDBC driver (via the 'serverTimezone' configuration property) to use a more specifc time zone value if you want to utilize time zone support. |
I changed the MySQL Driver reference as instructed by the error message:
29 30 31 | // Create instance of MySQLDriver. Class.forName ("com.mysql.cj.jdbc.Driver").newInstance(); conn = DriverManager.getConnection (url, username, password); |
After the change, I got the following error while retesting my little MySQL Java driver connection test program. Initially, I thought this required a change in the Java environment but that wasn’t it.
Cannot connect to database server: The server time zone value 'MDT' is unrecognized or represents more than one time zone. You must configure either the server or JDBC driver (via the 'serverTimezone' configuration property) to use a more specifc time zone value if you want to utilize time zone support. |
It required me to add the following line to my /etc/my.cnf configuration file, which synchronizes the database’s timezone with the operating system.
# Synchronize the MySQL clock with the computer system clock. default-time-zone='+00:00' |
Then, running my MySQL Driver connection test program worked like a charm. It returns the following:
Database connection established MySQLDriver Version [8.0.19] Database connection terminated |
Here’s the MySQL Connector Java code if you’d like to use the MySQLDriver.java file:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 | // Import classes. import java.sql.*; /* You can't include the following on Linux without raising an exception. */ // import com.mysql.jdbc.Driver; public class MySQLDriver { public MySQLDriver() { /* Declare variables that require explicit assignments because they're addressed in the finally block. */ Connection conn = null; Statement stmt = null; ResultSet rset = null; /* Declare other variables. */ String url; String username = "student"; String password = "student"; String database = "studentdb"; String hostname = "localhost"; String port = "3306"; String sql; /* Attempt a connection. */ try { // Set URL. url = "jdbc:mysql://" + hostname + ":" + port + "/" + database; // Create instance of MySQLDriver. Class.forName ("com.mysql.cj.jdbc.Driver").newInstance(); conn = DriverManager.getConnection (url, username, password); // Query the version of the database. sql = "SELECT version()"; stmt = conn.createStatement(); rset = stmt.executeQuery(sql); System.out.println ("Database connection established"); // Read row returns for one column. while (rset.next()) { System.out.println("MySQLDriver Version [" + rset.getString(1) + "]"); } } catch (SQLException e) { System.err.println ("Cannot connect to database server:"); System.out.println(e.getMessage()); } catch (ClassNotFoundException e) { System.err.println ("Cannot find MySQL driver class:"); System.out.println(e.getMessage()); } catch (InstantiationException e) { System.err.println ("Cannot instantiate class:"); System.out.println(e.getMessage()); } catch (IllegalAccessException e) { System.err.println ("Illegal access exception:"); System.out.println(e.getMessage()); } finally { if (conn != null) { try { rset.close(); stmt.close(); conn.close(); System.out.println ("Database connection terminated"); } catch (Exception e) { /* ignore close errors */ } } } } /* Unit test. */ public static void main(String args[]) { new MySQLDriver(); } } |
As always, I hope this helps those who encounter similar problems.
DBeaver for PostgreSQL
I’m migrating my database classes from the Oracle database to the PostgreSQL database. Using the Oracle Express Edition has always required a virtualized image because students use Windows and Mac OS. Also, the university doesn’t like my use of a virtualized image. Virtualization imposes incremental cost on students to have high end laptops.
The available Docker images don’t typically support the Oracle Express Edition. That means there are licensing implications tied to Oracle.
As a committee, we decided to use PostgreSQL as our new database platform. We opted to use PostgreSQL over MySQL because it supports arrays and stored procedures. PostgreSQL PL/pgSQL are a solid implementation of stored procedures. While MySQL supports a stored procedure language, it effectively discourages using it.
We use sqlplus, mysql, or psql Command-Line Interface (CLI) as the primary interface. That’s because CLI supports production-oriented deployment and DevOps practices. The Open Source DBeaver project provides a clean native GUI management console for PostgreSQL. DBeaver also supports other databases, which pgAdmin doesn’t.
You click the Download button from the DBeaver home page. It support Windows, Mac OS, and Linux distributions. I downloaded the Linux RPM package to my Downloads directory. You can run the RPM with the following syntax:
rpm -ivh dbeaver-ce-6.2.3-stable.x86_64.rpm |
It doesn’t check for dependencies. The installation is quick. You can verify that it’s installed with the following command:
which dbeaver |
It installs here:
/usr/bin/dbeaver |
If you failed to install the PostgreSQL JAR file, you’ll see the following error message:
It will look like the following on Linux:
You should have a user in the pg_hba.conf file, like the following:
host all all 127.0.0.1/32 trust |
As always, I hope this helps those solving problems.
Java and Postgres
I wanted to get Java working with PostgreSQL to test some GUI interfaces on Linux. Figuring out the necessary JAR file for the JDBC was my first hurdle. I found it was postgreSQL-42-2.5.jar file.
You can download it with the following command line:
wget https://jdbc.postgresql.org/download/postgresql-42.2.5.jar |
I downloaded it to a Java directory off the home/student directory. Then, I added the following CLASSPATH to local java.env environment file.
#!/usr/bin/bash # Set the Java CLASSPATH environment variable to include the JDBC jar file. export set CLASSPATH=/home/student/Java/postgresql-42.2.5.jar:. |
I sourced the postgresql-42.2.5.jar file and I wrote the following JavaTest.java program:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 | /* Import classes. */ import java.sql.Connection; import java.sql.DriverManager; import java.sql.ResultSet; import java.sql.SQLException; import java.sql.Statement; import java.util.logging.Level; import java.util.logging.Logger; public class JavaTest { public static void main(String[] args) { /* Set default strings for the connection. */ String url = "jdbc:postgresql://localhost:5432/videodb"; String user = "student"; String password = "student"; /* Try the connection and statement. */ try { Connection conn = DriverManager.getConnection(url, user, password); Statement st = conn.createStatement(); ResultSet rs = st.executeQuery("SELECT VERSION()"); if (rs.next()) { System.out.println(rs.getString(1)); } /* Close the result set and statement. */ rs.close(); st.close(); } catch (SQLException e) { Logger logger = Logger.getLogger(JavaTest.class.getName()); logger.log(Level.SEVERE, e.getMessage(), e); } } } |
I compiled the JavaTest.java program and tested it. It failed with the following error:
FATAL: Ident authentication failed for user - Unable to connect to PostgreSQL |
The failure occurred because I hadn’t allowed the connection in PostgreSQL’s pg_hba.conf file. I changed the following line in my pg_hba.conf file:
host all all 127.0.0.1/32 ident |
to
host all all 127.0.0.1/32 trust |
Then, I restarted the postgresql-11 service, like this:
systemctl restart postgresql-11.service |
The JavaTest program ran successfully and returned:
PostgreSQL 11.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 9.1.1 20190503 (Red Hat 9.1.1-1), 64-bit |
You can extend the logic to output a comma-separated value file by leveraging the JDBC help page, like this:
/* Import classes. */ import java.sql.Connection; import java.sql.DriverManager; import java.sql.ResultSet; import java.sql.ResultSetMetaData; import java.sql.SQLException; import java.sql.Statement; import java.util.logging.Level; import java.util.logging.Logger; /* Create class withonly a static method for runtime testing. */ public class JavaReturnSet { public static void main(String[] args) { /* Set default strings for the connection. */ String url = "jdbc:postgresql://localhost:5432/videodb"; String user = "student"; String password = "student"; /* Try the connection and statement. */ try { /* Set connection, statement, and result set. */ Connection conn = DriverManager.getConnection(url, user, password); Statement st = conn.createStatement(); /* Use the + to concatenate lines for query clarity. */ ResultSet rs = st.executeQuery("SELECT m.account_number\n" + ", CONCAT(c.last_name,', ',c.first_name)\n" + "FROM member m JOIN contact c\n" + "ON m.member_id = c.member_id"); /* Get query metadata for subsequent management of results. */ ResultSetMetaData rsmd = rs.getMetaData(); String line; while (rs.next()) { /* Initialize the line output for each row. */ line = ""; /* Process the columns. */ for (int i = 0; i < rsmd.getColumnCount(); i++) { if (rsmd.getColumnType(i + 1) == 12) { line = line + "\"" + rs.getString(i + 1) + "\""; } else { line = line + rs.getInt(i + 1); } /* Put a comma between output columns. */ if (i < rsmd.getColumnCount() - 1) { line = line + ","; } } System.out.println(line); } /* Close the result set and statement. */ rs.close(); st.close(); } catch (SQLException e) { Logger logger = Logger.getLogger(JavaReturnSet.class.getName()); logger.log(Level.SEVERE, e.getMessage(), e); } } } |
As always, I hope this helps those looking for a clean solution.
Postgres Print Debug Notes
A student asked how you print output from PL/pgSQL blocks. The student wanted to know if there was something like the following in Oracle’s PL/SQL programming language:
dbms_output.put_line('some string'); |
or, in Java programming the:
System.out.println("some string"); |
The RAISE NOTICE is the equivalent to these in Postgres PL/pgSQL, as shown in the following anonymous block:
do $$ BEGIN raise notice 'Hello World!'; END; $$; |
It prints:
NOTICE: Hello World! |
You can write a hello_world function as a named PL/pgSQL block:
CREATE FUNCTION hello_world() RETURNS text AS $$ DECLARE output VARCHAR(20); BEGIN /* Query the string into a local variable. */ SELECT 'Hello World!' INTO output; /* Return the output text variable. */ RETURN output; END $$ LANGUAGE plpgsql; |
You can call it with the following:
SELECT hello_world(); |
It prints:
hello_world -------------- Hello World! (1 row) |
Here’s a full test case with stored procedure in PL/pgSQL:
-- Drop the msg table. DROP TABLE msg; -- Create the msg table. CREATE TABLE msg ( comment VARCHAR(400) ); -- Transaction Management Example. DROP PROCEDURE IF EXISTS testing ( IN pv_one VARCHAR(30) , IN pv_two VARCHAR(10)); -- Transaction Management Example. CREATE OR REPLACE PROCEDURE testing ( IN pv_one VARCHAR(30) , IN pv_two VARCHAR(10)) AS $$ DECLARE /* Declare error handling variables. */ err_num TEXT; err_msg INTEGER; BEGIN /* Log actdual parameter values. */ INSERT INTO msg VALUES (pv_one||'.'||pv_two); EXCEPTION WHEN OTHERS THEN err_num := SQLSTATE; err_msg := SUBSTR(SQLERRM,1,100); RAISE NOTICE 'Trapped Error: %', err_msg; END $$ LANGUAGE plpgsql; do $$ DECLARE lv_one VARCHAR(30) := 'INDIVIDUAL'; lv_two VARCHAR(19) := 'R11-514-34'; BEGIN RAISE NOTICE '[%]', lv_one; RAISE NOTICE '[%]', lv_two; CALL testing( pv_one := lv_one, pv_two := lv_two ); END $$; -- Query any logged results. SELECT * FROM msg; |
It prints:
DROP TABLE
CREATE TABLE
DROP PROCEDURE
CREATE PROCEDURE
psql:fixed.sql:61: NOTICE: [INDIVIDUAL]
psql:fixed.sql:61: NOTICE: [R11-514-34]
DO
comment
-----------------------
INDIVIDUAL.R11-514-34
(1 row) |
I hope this helps those looking for a solution.
Cassandra on Fedora 30
The first thing to do with Fedora 30 is to check what part of Apache Cassandra is installed. You can use the following rpm command to determine that:
rpm -qa | grep cassandra |
My Fedora 30 returned the following values:
cassandra-java-libs-3.11.1-12.fc30.x86_64 cassandra-python2-cqlshlib-3.11.1-12.fc30.x86_64 cassandra-3.11.1-12.fc30.x86_64 python2-cassandra-driver-3.18.0-1.fc30.x86_64 |
Notably missing from the list of rpm list is the cassandra-server package. You install cassandra-server with the def utility:
dnf install -y cassandra-server |
You should get an installation log like the following for the cassandra-server package:
Display detailed console log →
Last metadata expiration check: 0:26:07 ago on Wed 11 Sep 2019 09:10:08 PM MDT. Package cassandra-3.11.1-12.fc30.x86_64 is already installed. Dependencies resolved. =========================================================================================================================== Package Architecture Version Repository Size =========================================================================================================================== Installing: cassandra-server x86_64 3.11.1-12.fc30 fedora 180 k Installing dependencies: sigar x86_64 1.6.5-0.20.git58097d9.fc27 fedora 76 k Transaction Summary =========================================================================================================================== Install 2 Packages Total download size: 255 k Installed size: 738 k Is this ok [y/N]: y Downloading Packages: (1/2): sigar-1.6.5-0.20.git58097d9.fc27.x86_64.rpm 131 kB/s | 76 kB 00:00 (2/2): cassandra-server-3.11.1-12.fc30.x86_64.rpm 233 kB/s | 180 kB 00:00 --------------------------------------------------------------------------------------------------------------------------- Total 116 kB/s | 255 kB 00:02 Running transaction check Transaction check succeeded. Running transaction test Transaction test succeeded. Running transaction Preparing : 1/1 Installing : sigar-1.6.5-0.20.git58097d9.fc27.x86_64 1/2 Running scriptlet: sigar-1.6.5-0.20.git58097d9.fc27.x86_64 1/2 Running scriptlet: cassandra-server-3.11.1-12.fc30.x86_64 2/2 Installing : cassandra-server-3.11.1-12.fc30.x86_64 2/2 Running scriptlet: cassandra-server-3.11.1-12.fc30.x86_64 2/2 Verifying : cassandra-server-3.11.1-12.fc30.x86_64 1/2 Verifying : sigar-1.6.5-0.20.git58097d9.fc27.x86_64 2/2 Installed: cassandra-server-3.11.1-12.fc30.x86_64 sigar-1.6.5-0.20.git58097d9.fc27.x86_64 Complete! |
Fedora Magazine has a great Get Started with Apache Cassandra on Fedora article on all the steps required to setup clusters. This article only covers creating and enabling the Cassandra service, and setting up a single node Cassandra instance.
You start Cassandra with the following command as the root user:
systemctl start cassandra |
You enable Cassandra with the following command as the root user:
systemctl enable cassandra |
It creates the following symlink:
Created symlink /etc/systemd/system/multi-user.target.wants/cassandra.service → /usr/lib/systemd/system/cassandra.service. |
You can connect to the Test cluster with the following command:
cqlsh |
You should see the following:
Connected to Test Cluster at 127.0.0.1:9042. [cqlsh 5.0.1 | Cassandra 3.11.1 | CQL spec 3.4.4 | Native protocol v4] Use HELP for help. |
You can see the options by typing the help command:
Documented shell commands: =========================== CAPTURE CLS COPY DESCRIBE EXPAND LOGIN SERIAL SOURCE UNICODE CLEAR CONSISTENCY DESC EXIT HELP PAGING SHOW TRACING CQL help topics: ================ AGGREGATES CREATE_KEYSPACE DROP_TRIGGER TEXT ALTER_KEYSPACE CREATE_MATERIALIZED_VIEW DROP_TYPE TIME ALTER_MATERIALIZED_VIEW CREATE_ROLE DROP_USER TIMESTAMP ALTER_TABLE CREATE_TABLE FUNCTIONS TRUNCATE ALTER_TYPE CREATE_TRIGGER GRANT TYPES ALTER_USER CREATE_TYPE INSERT UPDATE APPLY CREATE_USER INSERT_JSON USE ASCII DATE INT UUID BATCH DELETE JSON BEGIN DROP_AGGREGATE KEYWORDS BLOB DROP_COLUMNFAMILY LIST_PERMISSIONS BOOLEAN DROP_FUNCTION LIST_ROLES COUNTER DROP_INDEX LIST_USERS CREATE_AGGREGATE DROP_KEYSPACE PERMISSIONS CREATE_COLUMNFAMILY DROP_MATERIALIZED_VIEW REVOKE CREATE_FUNCTION DROP_ROLE SELECT CREATE_INDEX DROP_TABLE SELECT_JSON |
Here’s my script that creates Cassandra keyspace, which is more or less a database. You use the USE command to connect to the keyspace or database, like you would in MySQL. You do not have sequences in Cassandra because they’re not a good fit for a distributed architecture. Cassandra does not support a native procedural extension like relational databases. You must create User-defined functions (UDFs) by embedding the logic in Java.
This script does the following:
- Creates a keyspace
- Uses the keyspace
- Conditionally drops tables and functions
- Creates two tables
- Inserts data into the two tables
- Queries data from the tables
I also included a call to a UDF inside a query in two of the examples. One of the queries demonstrates how to return a JSON structure from a query. To simplify things and provide clarification of the scripts behaviors, the details are outlined below.
- The first segment of the script creates the keyspace, changes the scope to use the keyspace, conditionally drop tables, create tables, and insert values into the tables:
/* Create a keyspace in Cassandra, which is like a database in MySQL or a schema in Oracle. */ CREATE KEYSPACE IF NOT EXISTS student WITH REPLICATION = { 'class':'SimpleStrategy' ,'replication_factor': 1 } AND DURABLE_WRITES = true; /* Use the keyspace or connect to the database. */ USE student; /* Drop the member table from the student keyspace. */ DROP TABLE IF EXISTS member; /* Create a member table in the student keyspace. */ CREATE TABLE member ( member_number VARCHAR , member_type VARCHAR , credit_card_number VARCHAR , credit_card_type VARCHAR , PRIMARY KEY ( member_number )); /* Conditionally drop the contact table from the student keyspace. */ DROP TABLE IF EXISTS contact; /* Create a contact table in the student keyspace. */ CREATE TABLE contact ( contact_number VARCHAR , contact_type VARCHAR , first_name VARCHAR , middle_name VARCHAR , last_name VARCHAR , member_number VARCHAR , PRIMARY KEY ( contact_number )); /* Insert a row into the member table. */ INSERT INTO member ( member_number, member_type, credit_card_number, credit_card_type ) VALUES ('SFO-12345','GROUP','2222-4444-5555-6666','VISA'); /* Insert a row into the contact table. */ INSERT INTO contact ( contact_number, contact_type, first_name, middle_name, last_name, member_number ) VALUES ('CUS_00001','FAMILY','Barry', NULL,'Allen','SFO-12345'); /* Insert a row into the contact table. */ INSERT INTO contact ( contact_number, contact_type, first_name, middle_name, last_name, member_number ) VALUES ('CUS_00002','FAMILY','Iris', NULL,'West-Allen','SFO-12345'); /* Insert a row into the member table. */ INSERT INTO member ( member_number, member_type, credit_card_number, credit_card_type ) VALUES ('SFO-12346','GROUP','3333-8888-9999-2222','VISA'); /* Insert a row into the contact table. */ INSERT INTO contact ( contact_number, contact_type, first_name, middle_name, last_name, member_number ) VALUES ('CUS_00003','FAMILY','Caitlin','Marie','Snow','SFO-12346');
The following queries the member table:
/* Select all columns from the member table. */ SELECT * FROM member;
It returns the following:
member_number | credit_card_number | credit_card_type | member_type ---------------+---------------------+------------------+------------- SFO-12345 | 2222-4444-5555-6666 | VISA | GROUP SFO-12346 | 3333-8888-9999-2222 | VISA | GROUP
- Create a
concatenateUser-defined function (UDF) for Cassandra. The first step requires you to edit thecassandra.yamlfile, which you find in the/etc/cassandra/default.confdirectory. There is a single parameter that you need to edit, and it is theenable_user_defined_functionsparameter. By default the parameter is set tofalse, and you need to enable it to create UDFs.If you open the
cassandra.yamlfile as therootuser, you should find the parameter on line 987, like:983 984 985 986 987
# If unset, all GC Pauses greater than gc_log_threshold_in_ms will log at # INFO level # UDFs (user defined functions) are disabled by default. # As of Cassandra 3.0 there is a sandbox in place that should prevent execution of evil code. enable_user_defined_functions: false
After you make the edit, the
cassandra.yamlfile should look like this:983 984 985 986 987
# If unset, all GC Pauses greater than gc_log_threshold_in_ms will log at # INFO level # UDFs (user defined functions) are disabled by default. # As of Cassandra 3.0 there is a sandbox in place that should prevent execution of evil code. enable_user_defined_functions: true
After you make the change, you can create your own UDF. The following UDF formats the first, middle, and last name so there’s only one whitespace between the first and last name when there middle name value is null.
This type of function must use a
CALLED ON NULL INPUTclause in lieu of aRETURNS NULL ON NULL INPUTclause. The latter would force the function to return a null value if any one of the parameters were null./* Drop the concatenate function because a replace disallows changing a RETURNS NULL ON NULL INPUT with a CALLED ON NULL INPUT without raising an "89: InvalidRequest" exception. */ DROP FUNCTION concatenate; /* Create a user-defined function to concatenate names. */ CREATE OR REPLACE FUNCTION concatenate (first_name VARCHAR, middle_name VARCHAR, last_name VARCHAR) CALLED ON NULL INPUT RETURNS VARCHAR LANGUAGE java AS $$ /* Concatenate first and last names when middle name is null, and first, middle, and last names when middle name is not null. */ String name; /* Check for null middle name. */ if (middle_name == null) { name = first_name + " " + last_name; } else { name = first_name + " " + middle_name + " " + last_name; } return name; $$;
Query the values from the contact table with the UDF function in the SELECT-list:
/* Query the contact information. */ SELECT member_number , contact_number , contact_type , concatenate(first_name, middle_name, last_name) AS full_name FROM contact; |
It returns the following:
member_number | contact_number | contact_type | full_name ---------------+----------------+--------------+-------------------- SFO-12345 | CUS_00001 | FAMILY | Barry Allen SFO-12345 | CUS_00002 | FAMILY | Iris West-Allen SFO-12346 | CUS_00003 | FAMILY | Caitlin Marie Snow |
Query the values from the contact table with a JSON format:
/* Query the contact information and return in a JSON format. */ SELECT JSON contact_number , contact_type , concatenate(first_name, middle_name, last_name) AS full_name FROM contact; |
It returns the following:
[json] ------------------------------------------------------------------------------------------------- {"contact_number": "CUS_00001", "contact_type": "FAMILY", "full_name": "Barry Allen"} {"contact_number": "CUS_00002", "contact_type": "FAMILY", "full_name": "Iris West-Allen"} {"contact_number": "CUS_00003", "contact_type": "FAMILY", "full_name": "Caitlin Marie Snow"} |
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