How to compile jdk 1.5 generated .class/java file with lower jdk versions

 Cross-Compilation Options

By default, classes are compiled against the bootstrap and extension classes of the platform that javac shipped with. But javac also supports cross-compiling, where classes are compiled against a bootstrap and extension classes of a different Java platform implementation. It is important to use -bootclasspath and -extdirs when cross-compiling; see Cross-Compilation Example below.

-target version

Generate class files that will work on VMs with the specified version. The default is to generate class files to be compatible with the JDK 5 VM. When the -source 1.4 or lower option is used, the default target is 1.4. The versions supported by javac are:

1.1	Generate class files that will run on VMs in JDK 1.1 and later.
1.2	Generate class files that will run on VMs in JDK 1.2 and later, but will not run on 1.1 VMs.
1.3	Generate class files that will run on VMs in JDK 1.3 and later, but will not run on 1.1 or 1.2 VMs.
1.4	Generate class files that will run on VMs in JDK 1.4 and later, but will not run on 1.1, 1.2 or 1.3 VMs.
1.5	Generate class files that are compatible only with JDK 5 VMs.
5	Synonym for 1.5

-bootclasspath bootclasspath

Cross-compile against the specified set of boot classes. As with the user class path, boot class path entries are separated by semicolons (;) and can be directories, JAR archives, or ZIP archives.

-extdirs directories

Cross-compile against the specified extension directories. Directories is a semicolon-separated list of directories. Each JAR archive in the specified directories is searched for class files.

Cross-Compilation Example

Here we use javac to compile code that will run on a 1.4 VM.

% javac -target 1.4 -bootclasspath jdk1.4.2/lib/classes.zip \
             -extdirs "" OldCode.java

The -target 1.4 option ensures that the generated class files will be compatible with 1.4 VMs. By default, javac compiles for JDK 5.The Java 2 SDK's javac would also by default compile against its own bootstrap classes, so we need to tell javac to compile against JDK 1.4 bootstrap classes instead. We do this with -bootclasspath and -extdirs. Failing to do this might allow compilation against a Java 2 Platform API that would not be present on a 1.4 VM and would fail at runtime.

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 it generally occurs if a higher JDK version  is used to compile the source file and  a lower JDK version is used to run the program. for example if you compile your java source file in JDK 1.5 and you will try to run it on JDK 1.4 you will get error


UnSupportedClassVersionError will not tell you for which class it’s coming. So if you are using multiple third party jars in your application you find that it comes at a particular part when JVM tries to load a class from a particular jar. anyway we all know that latest version of JDK is 1.6 so maximum version of class filecould be generated by JDK 6, so by using JDK 6 we can solve UnSupportedClassVersionError, but many times its not easy to just move to higher JDK version. So I would suggest:

1) Find out due to which jar or class file this UnSupportedClassVersionError is coming?

2) Try to compile source code of that jar with the JDK version you are using to run your program, if source is available.

3) If you don't have source try to find the compatible version of that library.

4) Increase the JRE version you are using to run your program.

 

Following are the major version of class file format in standard JDK environment.

JDK 1.1 = 45
JDK 1.2 = 46
JDK 1.3 = 47
JDK 1.4 = 48
JDK 1.5 = 49
JDK 1.6 = 50

 

 

1) If you encounter UnSupportedClassVersionError, check the JRE version you are using to run program and switch to higher version for quick solution.
2) java.lang.UnsupportedClassVersionError is derived from java.lang.LinkageError, so it will not be detected in compile time and it will only come on runtime, precisely when JVM tries to load a class.
3) Class file format which is identified using major version and minor version. Class file format is assigned when you compile source file and its depends on JDK version used to compile.
4) Its always best practice to use same version of java for compilation and execution to avoid any chance of UnSupportedClassVersionError.
5) UnSupportedClassVersionError is not related to java classpath , so don't confuse this with NoClassDefFoundError or ClassNotFoundException.