目标

假如有一个GoodChild类，内容如下：

public class GoodChild {
    public String name;
    public int age;

    public GoodChild(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

预期目标是生成一个HelloWorld类：

public class HelloWorld {
    public void test() {
        GoodChild child = new GoodChild("Lucy", 8);
    }
}

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", "()V", null, null);
            mv2.visitCode();
            mv2.visitTypeInsn(NEW, "sample/GoodChild");
            mv2.visitInsn(DUP);
            mv2.visitLdcInsn("Lucy");
            mv2.visitIntInsn(BIPUSH, 8);
            mv2.visitMethodInsn(INVOKESPECIAL, "sample/GoodChild", "<init>", "(Ljava/lang/String;I)V", false);
            mv2.visitVarInsn(ASTORE, 1);
            mv2.visitInsn(RETURN);
            mv2.visitMaxs(4, 2);
            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");
        m.invoke(obj);
    }
}

Frame的变化

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

public void test();
  Code:
     0: new           #11                 // class sample/GoodChild
     3: dup
     4: ldc           #13                 // String Lucy
     6: bipush        8
     8: invokespecial #16                 // Method sample/GoodChild."<init>":(Ljava/lang/String;I)V
    11: astore_1
    12: return

该方法对应的Frame变化情况如下：

test()V
[sample/HelloWorld] []
[sample/HelloWorld] [uninitialized_sample/GoodChild]
[sample/HelloWorld] [uninitialized_sample/GoodChild, uninitialized_sample/GoodChild]
[sample/HelloWorld] [uninitialized_sample/GoodChild, uninitialized_sample/GoodChild, java/lang/String]
[sample/HelloWorld] [uninitialized_sample/GoodChild, uninitialized_sample/GoodChild, java/lang/String, int]
[sample/HelloWorld] [sample/GoodChild]
[sample/HelloWorld, sample/GoodChild] []
[] []

小结

• 从Instruction的角度来讲，创建对象的指令集合：
  • new
  • dup
  • invokespecial
• 从Frame的角度来讲，在创建新对象的时候，执行new指令之后，它是uninitialized状态，执行invokespecial指令之后，它是一个“合格”的对象。