"javac -source 10 -target 10" - Lower Java Version

Q

How to lower the Java version in the bytecode? I want to generate class files for an older Java environment.

✍: FYIcenter

A

If you want generate class files for an older Java environment, you need to use "-source x" and "-target x" options in the "javac" command to compile the Java source code.

1. Check the default version of Java compiler:

> javac -version

javac 15

2. Compile HelloWorldFrame.java used in the last tutorial into a bytecode file with the default version:

> javac HelloWorldFrame.java 

3. Check the version of the bytecode. Bytecode major version 59 maps to Java 15.

> javap -v HelloWorldFrame.class | grep "major version" 
  major version: 59

4. Compile HelloWorldFrame.java again for Java 10:

> javac -source 10 -target 10 HelloWorldFrame.java 
warning: [options] system modules path not set in conjunction with -source 10
1 warning

5. Check the version of the bytecode again. Bytecode major version 54 maps to Java 10.

> javap -v HelloWorldFrame.class | grep major 
  major version: 54

This bytecode can be executed in Java 10 environment.

 

⇒ "javap jar:file:" - Specify Bytecode in JAR

⇐ "javap -v" - Major Version Code

⇑ JDK "javap" Command

⇑⇑ Java Bytecode Tools

2021-09-09, 168👍, 0💬