Java ASM 需要帮助

Question

我使用 Java ASM 编写了一个简单的程序来生成通过编译以下类生成的字节码。

public class Main {
    public static void main(String[] args) {
        System.out.println("Test");
    }
}

下面给出了我编写的为此类生成字节码的代码。

public class CodeGenerator {

    public void generateClass()
    {
        ClassWriter cw=new ClassWriter(Opcodes.NULL);
        FieldVisitor fv;
        MethodVisitor mv;
        cw.visit(Opcodes.V1_1, Opcodes.ACC_PUBLIC, "Main", null, "java/lang/Object", null);
        mv=cw.visitMethod(Opcodes.ACC_PUBLIC, "<init>", "()V", null, null);
        mv.visitVarInsn(Opcodes.AALOAD, 0);
        mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
        mv.visitInsn(Opcodes.RETURN);
        mv.visitMaxs(1, 1);
        mv.visitEnd();
        mv=cw.visitMethod(Opcodes.ACC_PUBLIC+ Opcodes.ACC_STATIC, "Main", "([Ljava/lang/String;)V", null, null);
        mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream");
        mv.visitLdcInsn("Test");
        mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V");
        mv.visitInsn(Opcodes.RETURN);
        mv.visitMaxs(2, 1);
        mv.visitEnd();
        cw.visitEnd();
        this.WriteClass(cw);
    }
    public void WriteClass(ClassWriter cw){
        FileOutputStream fos;
        try{
            fos = new FileOutputStream("E:\\Acadamic\\Final year project\\ASM\\Main.class");
            fos.write(cw.toByteArray());
            fos.close();
        }
        catch (IOException ex){
            Logger.getLogger(CodeGenerator.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

编译成功，当我尝试运行它时，出现以下错误。

Exception in thread "main" java.lang.NoClassDefFoundError: main/class
Caused by: java.lang.ClassNotFoundException: main.class
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: main.class.  Program will exit.

任何人都可以帮助我避免此错误。如果有人可以解释错误的原因以及如何避免它，请善意地帮助我。

谢谢。

后续:

当我像 java Main 一样运行它时，会出现以下错误。

Exception in thread "main" java.lang.ClassFormatError: Field "out" in class Main
 has illegal signature "Ljava/io/PrintStream"
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClassCond(Unknown Source)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$000(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: Main.  Program will exit.
 

Answer 1

我认为您的错误不是由 ASM 引起的。看来您只是错误地调用了 java 。你应该这样调用它:

java Main

My guess is that you called it like this:

java Main.class

Update

There are three problems with your code.

Problem #1

The ClassWriter constructor accepts a bit mask of options. You passed it Opcodes.NULL. Although this may seem correct, you actually want to pass it 0 (Opcodes.NULL means something different).

ClassWriter cw=new ClassWriter(0);

问题#2

您正在构造函数中调用 AALOAD，而不是 ALOAD。 AALOAD 指令加载数组元素，而ALOAD 加载局部变量。

mv.visitVarInsn(Opcodes.ALOAD, 0);

问题#3

在您的Main方法中，您忘记了Ljava/io/PrintStream后面的分号:

mv.visitFieldInsn(
    Opcodes.GETSTATIC,
    "java/lang/System",
    "out",
    "Ljava/io/PrintStream;" // Notice the semicolon
  );

您可以通过利用 ASM 的 Type 类来减轻自己的痛苦:

mv.visitFieldInsn(
    Opcodes.GETSTATIC,
    "java/lang/System",
    "out",
    Type.getObjectType("java/io/PrintStream").getDescriptor()
  );