如何在运行时发出和执行Java字节码？

Question

我正在用Java为具有特定脚本功能的领域特定语言编写解释器。我已经实现了解析器，现在需要做一个后端。为此，我正在考虑编写自己的解释器（使用抽象语法树或某些自定义字节码）或目标JVM（在运行时发出并执行Java字节码）。

对此领域有更多经验的人可以说针对JVM的方法可行吗？您建议使用哪些库来发出Java字节码？

Answer 1

这是使用ObjectWeb ASM（我推荐的一个库）构成的一个有效的“ hello world” ：

package hello;

import java.lang.reflect.Method;

import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class HelloWorldASM implements Opcodes {
    public static byte[] compile(String name) {
        ClassWriter cw = new ClassWriter(0);
        MethodVisitor mv;

        cw.visit(V1_6, ACC_PUBLIC + ACC_SUPER, "hello/HelloWorld", null,
                "java/lang/Object", null);

        cw.visitSource("HelloWorld.java", null);

        {
            mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(4, l0);
            mv.visitVarInsn(ALOAD, 0);
            mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>",
                    "()V");
            mv.visitInsn(RETURN);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLocalVariable("this", "Lhello/HelloWorld;", null, l0, l1,
                    0);
            mv.visitMaxs(1, 1);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC + ACC_STATIC, "main",
                    "([Ljava/lang/String;)V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(7, l0);
            mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out",
                    "Ljava/io/PrintStream;");
            mv.visitLdcInsn(String.format("Hello, %s!", name));
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println",
                    "(Ljava/lang/String;)V");
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLineNumber(8, l1);
            mv.visitInsn(RETURN);
            Label l2 = new Label();
            mv.visitLabel(l2);
            mv.visitLocalVariable("args", "[Ljava/lang/String;", null, l0, l2,
                    0);
            mv.visitMaxs(2, 1);
            mv.visitEnd();
        }
        cw.visitEnd();

        return cw.toByteArray();
    }

    public static class DynamicClassLoader extends ClassLoader {
        public Class<?> define(String className, byte[] bytecode) {
            return super.defineClass(className, bytecode, 0, bytecode.length);
        }
    };

    public static void main(String[] args) throws Exception {
        DynamicClassLoader loader = new DynamicClassLoader();
        Class<?> helloWorldClass = loader.define("hello.HelloWorld",
                compile("Test"));
        Method method = helloWorldClass.getMethod("main", String[].class);
        method.invoke(null, (Object) new String[] {});
    }
}

为了生成代码，我发现了非常有用的Eclipse插件字节码大纲。尽管您可以像这样使用ASMifier（包含在ASM中）：

ClassReader cr = new ClassReader(new FileInputStream("HelloWorld.class"));
cr.accept(new ASMifierClassVisitor(new PrintWriter(System.out)), 0);

在运行时，如果需要获取所Class创建类的对象，则可以通过扩展类加载器并发布（例如，通过其他方法）将该defineClass方法并以字节数组形式提供类来加载类，如例。

您还可以使用接口来处理创建的类，例如以下示例：

package hello;

import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class HelloWorldPlugin implements Opcodes {
    public static interface Plugin {
        void sayHello(String name);
    }

    public static byte[] compile() {

        ClassWriter cw = new ClassWriter(0);
        MethodVisitor mv;

        cw.visit(V1_6, ACC_PUBLIC + ACC_SUPER, "hello/MyClass", null,
                "java/lang/Object",
                new String[] { "hello/HelloWorldPlugin$Plugin" });

        cw.visitInnerClass("hello/HelloWorldPlugin$Plugin",
                "hello/HelloWorldPlugin", "Plugin", ACC_PUBLIC + ACC_STATIC
                        + ACC_ABSTRACT + ACC_INTERFACE);

        {
            mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(5, l0);
            mv.visitVarInsn(ALOAD, 0);
            mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>",
                    "()V");
            mv.visitInsn(RETURN);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLocalVariable("this", "Lhello/MyClass;", null, l0, l1, 0);
            mv.visitMaxs(1, 1);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC, "sayHello",
                    "(Ljava/lang/String;)V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(9, l0);
            mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out",
                    "Ljava/io/PrintStream;");
            mv.visitTypeInsn(NEW, "java/lang/StringBuilder");
            mv.visitInsn(DUP);
            mv.visitLdcInsn("Hello, ");
            mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder",
                    "<init>", "(Ljava/lang/String;)V");
            mv.visitVarInsn(ALOAD, 1);
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder",
                    "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;");
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder",
                    "toString", "()Ljava/lang/String;");
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println",
                    "(Ljava/lang/String;)V");
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLineNumber(10, l1);
            mv.visitInsn(RETURN);
            Label l2 = new Label();
            mv.visitLabel(l2);
            mv.visitLocalVariable("this", "Lhello/MyClass;", null, l0, l2, 0);
            mv.visitLocalVariable("name", "Ljava/lang/String;", null, l0, l2,
                    1);
            mv.visitMaxs(4, 2);
            mv.visitEnd();
        }
        cw.visitEnd();

        return cw.toByteArray();
    }

    public static class DynamicClassLoader extends ClassLoader {
        public DynamicClassLoader(ClassLoader parent) {
            super(parent);
        }

        public Class<?> define(String className, byte[] bytecode) {
            return super.defineClass(className, bytecode, 0, bytecode.length);
        }
    };

    public static void main(String[] args) throws Exception {
        DynamicClassLoader loader = new DynamicClassLoader(Thread
                .currentThread().getContextClassLoader());
        Class<?> helloWorldClass = loader.define("hello.MyClass", compile());
        Plugin plugin = (Plugin) helloWorldClass.newInstance();
        plugin.sayHello("Test");
    }
}

玩得开心。

PS：如果不够清晰，我可以在代码中添加注释。我不是因为答案已经太长了。不过，我对您的建议是尝试对其进行调试。