Replacing statically referenced method in Java

问题:

I have a class like below with a method that just returns a String, but I want to modify what it returns from another class, without hardcoding it myself.

public class Name {
    public static String getName() {
        return "MyName";
    }
}

Is there any way to do this? I tried BCEL but that didn't seem to change the return value.

Edit: This is for a mod. I'm trying to make it completely independant from the existing code, by not modifying it.

Thanks.

回答1:

Are you sure you've tried BCEL? I created a fully-working example here.

JavaClass target;
try {
  target = Repository.lookupClass("Target");
} catch (final ClassNotFoundException ex) {
  throw new RuntimeException("unable to resolve Target", ex);
}
final ClassGen targetGen = new ClassGen(target);
final ConstantPoolGen pool = targetGen.getConstantPool();
final ConstantMethodref ref = (ConstantMethodref) pool.getConstant(
    pool.lookupMethodref("Name", "getName", "()Ljava/lang/String;"));
ref.setClassIndex(pool.lookupClass("Target"));
ref.setNameAndTypeIndex(pool.addNameAndType("$Name$getName", "()Ljava/lang/String;"));
final InstructionList code = new InstructionList();
final InstructionFactory factory = new InstructionFactory(targetGen, pool);
code.append(factory.createConstant("overriden-name"));
code.append(factory.createReturn(Type.STRING));
code.setPositions();
final MethodGen methodGen = new MethodGen(
    Constants.ACC_PRIVATE | Constants.ACC_SYNTHETIC | Constants.ACC_STATIC,
    Type.STRING, new Type[0], new String[0], "$Name$getName", "Target",
    code, pool);
methodGen.setMaxLocals(0);
methodGen.setMaxStack(1);
targetGen.addMethod(methodGen.getMethod());
try {
  targetGen.getJavaClass().dump("Target.class");
} catch (final IOException ex) {
  throw new RuntimeException("unable to save Target", ex);
}

C:\dev\scrap>javac Target.java
C:\dev\scrap>java Target
original-name
C:\dev\scrap>javac -cp .;bcel-6.0.jar Instrumenter.java
C:\dev\scrap>java -cp .;bcel-6.0.jar Instrumenter
C:\dev\scrap>java Target
overriden-name

回答2:

You can pass parameter into method

 public class Main {
        getName("newName")
    }

public class Name {
    public static String getName(String name) {
        return name;
    }
}

回答3:

You can try to add a -javaagent agent which will use something like asm or bcel to modify the bytecode of Name class such that the static method will return another string. Many mock testing frameworks - for e.g. powermock or jmockit - can do this.

EDIT: Here is some sample code to get you started. This is a javaagent which can modify a given public static method which returns a String to return another constant String. For e.g:

public class TestMain
{
    public static void main(String[] args)
    {
        System.out.println(Name.getName());
    }
}

class Name
{
    public static String getName()
    {
        return "ORIGINAL";
    }
}

$ java -cp build/libs/bciex.jar mycompany.myapp.TestMain
ORIGINAL
$ java -cp build/libs/bciex.jar -javaagent:build/libs/bciex.jar="mycompany.myapp.TestMain|getName|SOME_STRING" mycompany.myapp.TestMain
Agent loaded; will modify [getName] method of classes in [mycompany.myapp.TestMain] to return [SOME_STRING]
SOME_STRING
$