【发布时间】：2014-05-05 20:27:35
【问题描述】：
我想用新内容（sample.class:sayHello 方法）替换方法体，然后执行 sample.class。原来的sayHelo声明是：
public int sayHello(String args){
}
我想将它的主体修改为：
System.out.println("sxu says: hello world!");
return 1;
但是示例执行结果抛出异常：
Exception in thread "main" java.lang.VerifyError: Operand stack overflow
Exception Details:
Location:
code/sxu/demo/data/sample.sayHello(Ljava/lang/String;)I @4: ldc
Reason:
Exceeded max stack size.
Current Frame:
bci: @4
flags: { }
locals: { 'code/sxu/demo/data/sample', 'java/lang/String' }
stack: { 'code/sxu/demo/data/sample', 'java/io/PrintStream' }
Bytecode:
0000000: 2ab2 0028 122a b600 1704 acb2 0028 2bb6
0000010: 0017 04ac
我使用 asm 工具，我的代码中有三个 java 类：
public class Adapt extends ClassLoader {
@Override
protected synchronized Class<?> loadClass(final String name,
final boolean resolve) throws ClassNotFoundException {
if (name.startsWith("java.")) {
return super.loadClass(name, resolve);
} else {
System.err.println("Adapt: loading class '" + name
+ "' with on the fly adaptation");
}
// gets an input stream to read the bytecode of the class
String resource = name.replace('.', '/') + ".class";
InputStream is = getResourceAsStream(resource);
byte[] b;
// adapts the class on the fly
try {
ClassReader cr = new ClassReader(is);
ClassWriter cw = new ClassWriter(0);
ClassVisitor cv = new ClassAdapter(cw);
cr.accept(cv, 0);
b = cw.toByteArray();
} catch (Exception e) {
throw new ClassNotFoundException(name, e);
}
FileOutputStream fos = new FileOutputStream("/tmp/"+resource.substring(resource.lastIndexOf("/")) );
fos.write(b);
fos.close();
// returns the adapted class
return defineClass(name, b, 0, b.length);
}
public static void main(String[] args) {
// loads the application class (in args[0]) with an Adapt class loader
ClassLoader loader = new Adapt();
Class<?> c;
try {
c = loader.loadClass(args[0]);
// calls the 'main' static method of this class with the
// application arguments (in args[1] ... args[n]) as parameter
Method m = c.getMethod("main", new Class<?>[] { String[].class });
String[] applicationArgs = new String[args.length - 1];
System.arraycopy(args, 1, applicationArgs, 0, applicationArgs.length);
m.invoke(null, new Object[] { applicationArgs });
}
}
public class ClassAdapter extends ClassVisitor {
private String owner;
public ClassAdapter(ClassVisitor cv) {
super(Opcodes.ASM4, cv);
}
@Override
public void visit(final int version, final int access, final String name,
final String signature, final String superName,
final String[] interfaces) {
owner = name;
super.visit(version, access, name, signature, superName, interfaces);
}
@Override
public FieldVisitor visitField(final int access, final String name,
final String desc, final String signature, final Object value) {
FieldVisitor fv = super
.visitField(access, name, desc, signature, value);
if ((access & Opcodes.ACC_STATIC) == 0) { ...
}
return fv;
}
@Override
public MethodVisitor visitMethod(final int access, final String name,
final String desc, final String signature, final String[] exceptions) {
System.out.println("calling method name: "+ name);
MethodVisitor mv = cv.visitMethod(access, name, desc, signature,
exceptions);
//Rewrite sayHello method
if(name.equals("sayHello")){
mv = new MethodModifierAdapter(mv);
return mv;
/*
mv.visitCode();
mv.visitVarInsn(Opcodes.ALOAD, 0);
mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out",
"Ljava/io/PrintStream;");
// pushes the "Hello World!" String constant
mv.visitLdcInsn("Sxu says: Hello world!");
// invokes the 'println' method (defined in the PrintStream class)
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println",
"(Ljava/lang/String;)V");
mv.visitInsn(Opcodes.ICONST_1);
mv.visitInsn(Opcodes.IRETURN);
// this code uses a maximum of two stack elements and two local
// variables
mv.visitMaxs(5, 5);
mv.visitEnd();
*/
}
return mv == null ? null : new TraceFieldCodeAdapter(Opcodes.ASM4, mv, owner);
}
}
public class MethodModifierAdapter extends MethodVisitor implements Opcodes {
boolean _modified ;
public MethodModifierAdapter(MethodVisitor mv) {
super(Opcodes.ASM4, mv);
_modified = false;
}
@Override
public void visitCode() {
if(!_modified){
_modified = true;
mv.visitCode();
mv.visitVarInsn(Opcodes.ALOAD, 0);
mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out",
"Ljava/io/PrintStream;");
// pushes the "Hello World!" String constant
mv.visitLdcInsn("Sxu says: Hello world!");
// invokes the 'println' method (defined in the PrintStream class)
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println",
"(Ljava/lang/String;)V");
mv.visitInsn(Opcodes.ICONST_1);
mv.visitInsn(Opcodes.IRETURN);
// this code uses a maximum of two stack elements and two local
// variables
mv.visitMaxs(5, 5);
mv.visitEnd();
}
}
}
在 ClassAdaptor::visitMethod 中，我创建了一个新的 MethodModifierAdapter。 MethodModifierAdapter 的 visitCode 然后重写方法体。
新建sample.class的javap结果为：
public int sayHello(java.lang.String);
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: getstatic #40 // Field java/lang/System.out:Ljava/io/PrintStream;
4: ldc #42 // String Sxu says: Hello world!
6: invokevirtual #23 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
9: iconst_1
10: ireturn
11: getstatic #40 // Field java/lang/System.out:Ljava/io/PrintStream;
14: aload_1
15: invokevirtual #23 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
18: iconst_1
19: ireturn
LocalVariableTable:
Start Length Slot Name Signature
11 9 0 this Lcode/sxu/demo/data/sample;
11 9 1 args Ljava/lang/String;
LineNumberTable:
line 12: 11
line 13: 18
MethodModifierAdapter::visitcode 根据其内容更新方法内容。
我猜我使用 ASM API 的方式有些错误，但不清楚是哪一个..
【问题讨论】：
标签： java bytecode java-bytecode-asm bytecode-manipulation