如何在运行时发出并执行 Java 字节码？

Question

我正在用 Java 为特定于领域的语言编写一个解释器，并具有一些脚本功能。我已经实现了一个解析器，现在需要做一个后端。为此，我正在考虑编写自己的解释器（使用抽象语法树或一些自定义字节码）或目标 JVM（在运行时发出并执行 Java 字节码）。

在这方面有更多经验的人能否说一下针对 JVM 的方法有多可行以及您建议使用哪些库来生成 Java 字节码？

Answer 1

这是一个使用 ObjectWeb ASM （我推荐的库）制作的工作“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[] {});
    }
}

生成代码，我发现非常有用的 Bytecode Outline for Eclipse 插件。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，如果您需要获取创建的类的 Class 对象，您可以通过扩展类加载器并发布 (通过另一种方法（例如）defineClass 方法并将类作为字节数组提供，如示例中所列。

您还可以使用接口处理创建的类，如本示例所示：

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：如果代码不够清晰，我可以添加注释。我没有，因为答案已经太长了。尽管如此，我对你的建议是尝试调试它。

Answer 2

我建议您看一下这些库：

• CGLIB
• CGLIB apache.org/bcel/" rel="noreferrer">BCEL
• asm
• Javassist

Answer 3

从不同的角度我问你是否考虑过使用XText。它旨在使您能够创建 DSL、具有代码完成功能的代码编辑器、编译器、代码生成器等。我认为它真的很酷，并且有一个很好的文档。值得一看。您可以基于它轻松地为您的 DSL 创建编译器。