CODEWITHSUNDEEP
javamavenantbytecode
J Fabian Meier
J Fabian Meier

Reputation: 35843

Byte code difference in #-values after change Ant to Maven

I changed the compilation of a project from Ant to Maven. We still use Java 1.7.0_80. The ByteCode of all classes stayed the same except for a Comparator class. I disassembled the class with javap -c. The only differences are in #-values, e.g.:

before: invokevirtual #2

after: invokevirtual #29

What do these #-values mean? Is this a functional difference or just a difference in naming?

EDIT: The first method in Bytecode before and after:

  public de.continentale.kvrl.model.comp.KvRLRechBetragEuroComparator();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        


  public de.continentale.kvrl.model.comp.KvRLRechBetragEuroComparator();
    Code:
       0: aload_0       
       1: invokespecial #21                 // Method java/lang/Object."<init>":()V
       4: return

Upvotes: 2

Views: 144

Answers (2)

Heisenbug
Heisenbug

Reputation: 484

Changes are because ANT keeps debug mode to false for its javac command while maven keeps it true.

If you want ANT to produce same .class file or same bytecode as javac in cmd or maven, you need to add one more flag into javac of ANT script.

debug = "true"

Upvotes: 0

Erwin Bolwidt
Erwin Bolwidt

Reputation: 31299

The #29 refers to an index in the constant table that describes the method being invoked. It would have been better if you had pasted the full output of javap -c, because the comment at the end of the line indicates the actual method descriptor.

Example of javap -c on an invocation of an instance method test in class Z:

   7: invokevirtual #19                 // Method snippet/Z.test:()V

You can ignore the # number, as long as the method descriptor in the comment is the same.

Why

As to the "why" question, it is very likely caused by the debug in the ant and maven compiler task/plugin settings.

By default, the maven-compiler-plugin compiles with the debug configuration option set to true, which will attach among other the name of the source file, and thing like line number tables and local variable names.

https://maven.apache.org/plugins/maven-compiler-plugin/compile-mojo.html

By default, the ant java compiler has this set to false.

https://ant.apache.org/manual/Tasks/javac.html

To compare and check, set the debug parameter of the Ant Javac task to true and see if that makes the generated .class files the same.

Upvotes: 2

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