目标

public class HelloWorld {
    public void test(int a, int b) {
        int val = Math.max(a, b); // 对static方法进行调用
        System.out.println(val);  // 对non-static方法进行调用
    }
}

编码实现

import lsieun.utils.FileUtils;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodVisitor;

import static org.objectweb.asm.Opcodes.*;

public class HelloWorldGenerateCore {
    public static void main(String[] args) throws Exception {
        String relative_path = "sample/HelloWorld.class";
        String filepath = FileUtils.getFilePath(relative_path);

        // (1) 生成byte[]内容
        byte[] bytes = dump();

        // (2) 保存byte[]到文件
        FileUtils.writeBytes(filepath, bytes);
    }

    public static byte[] dump() throws Exception {
        // (1) 创建ClassWriter对象
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);

        // (2) 调用visitXxx()方法
        cw.visit(V1_8, ACC_PUBLIC + ACC_SUPER, "sample/HelloWorld", null, "java/lang/Object", null);

        {
            MethodVisitor mv1 = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
            mv1.visitCode();
            mv1.visitVarInsn(ALOAD, 0);
            mv1.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
            mv1.visitInsn(RETURN);
            mv1.visitMaxs(1, 1);
            mv1.visitEnd();
        }

        {
            MethodVisitor mv2 = cw.visitMethod(ACC_PUBLIC, "test", "(II)V", null, null);
            mv2.visitCode();
            mv2.visitVarInsn(ILOAD, 1);
            mv2.visitVarInsn(ILOAD, 2);
            mv2.visitMethodInsn(INVOKESTATIC, "java/lang/Math", "max", "(II)I", false);
            mv2.visitVarInsn(ISTORE, 3);
            mv2.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
            mv2.visitVarInsn(ILOAD, 3);
            mv2.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(I)V", false);
            mv2.visitInsn(RETURN);
            mv2.visitMaxs(2, 4);
            mv2.visitEnd();
        }

        cw.visitEnd();

        // (3) 调用toByteArray()方法
        return cw.toByteArray();
    }
}

验证结果

import java.lang.reflect.Method;

public class HelloWorldRun {
    public static void main(String[] args) throws Exception {
        Class<?> clazz = Class.forName("sample.HelloWorld");
        Object obj = clazz.newInstance();

        Method m = clazz.getDeclaredMethod("test", int.class, int.class);
        m.invoke(obj, 10, 20);
    }
}

Frame的变化

对于HelloWorld类中test()方法对应的Instruction内容如下：

public void test(int, int);
  Code:
     0: iload_1
     1: iload_2
     2: invokestatic  #21                 // Method java/lang/Math.max:(II)I
     5: istore_3
     6: getstatic     #27                 // Field java/lang/System.out:Ljava/io/PrintStream;
     9: iload_3
    10: invokevirtual #33                 // Method java/io/PrintStream.println:(I)V
    13: return

该方法对应的Frame变化情况如下：

test(II)V
[sample/HelloWorld, int, int] []
[sample/HelloWorld, int, int] [int]
[sample/HelloWorld, int, int] [int, int]
[sample/HelloWorld, int, int] [int]
[sample/HelloWorld, int, int, int] []
[sample/HelloWorld, int, int, int] [java/io/PrintStream]
[sample/HelloWorld, int, int, int] [java/io/PrintStream, int]
[sample/HelloWorld, int, int, int] []
[] []

小结

• 从Instruction的角度来讲，调用static方法是使用invokestatic指令，调用non-static方法一般使用invokevirtual指令。
• 从Frame的角度来讲，实现方法的调用，需要先将this变量和方法接收的参数放到operand stack上。