我正在用 Java 为具有一些脚本功能的特定领域语言编写解释器。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(使用抽象语法树或使用一些自定义字节码)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以 JVM 为目标的方法的可行性以及您推荐使用哪些库来发出 Java 字节码？
最佳答案
这是一个使用 ObjectWeb ASM 制作的“hello world”。 (我推荐的图书馆):
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[] {});
}
}
要生成代码，我发现非常有用Bytecode Outline for Eclipse插入。虽然您可以像这样使用 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，如果您需要为创建的类获取 Class 对象，您可以通过扩展类加载器并发布(例如通过另一种方法) 来加载您的类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(interface)处理创建的类，如下例所示:
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:如果不够清楚，我可以在代码中添加注释。我没有，因为答案已经太长了。不过，我给你的建议是尝试调试它。
关于java - 如何在运行时发出和执行 Java 字节码？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/4016305/