接上篇文章，操作字节码的框架有很多Javassist，ASM，BCEL等，这里我用ASM来举例。在了解字节码组成后，很容易通过ASM构建一个Class出来，代码如下：
package classloader;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
/**
* Created by dorole.com on 2016/6/13.
*/
public class HelloClassGeneratorTest {
public static void main(String[] args) throws IOException {
ClassWriter classWriter = new ClassWriter(0);
classWriter.visit(Opcodes.V1_7, Opcodes.ACC_PUBLIC, "Hello", null, "java/lang/Object", null);
// 构造方法
MethodVisitor constructorMethod = classWriter.visitMethod(Opcodes.ACC_PUBLIC, "<init>", "()V", null, null);
constructorMethod.visitCode();
constructorMethod.visitVarInsn(Opcodes.ALOAD, 0);
constructorMethod.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
constructorMethod.visitInsn(Opcodes.RETURN);
constructorMethod.visitMaxs(1, 1);
constructorMethod.visitEnd();
// sayHello方法
MethodVisitor helloMethod = classWriter.visitMethod(Opcodes.ACC_PUBLIC, "sayHello", "(Ljava/lang/String;)Ljava/lang/String;", null, null);
helloMethod.visitCode();
helloMethod.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
helloMethod.visitTypeInsn(Opcodes.NEW, "java/lang/StringBuilder");
helloMethod.visitInsn(Opcodes.DUP);
helloMethod.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "()V", false);
helloMethod.visitVarInsn(Opcodes.ALOAD, 0);
helloMethod.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/Object", "getClass", "()Ljava/lang/Class;", false);
helloMethod.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/Class", "getName", "()Ljava/lang/String;", false);
helloMethod.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
helloMethod.visitLdcInsn(" -> ");
helloMethod.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
helloMethod.visitVarInsn(Opcodes.ALOAD, 1);
helloMethod.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
helloMethod.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false);
helloMethod.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
helloMethod.visitVarInsn(Opcodes.ALOAD, 1);
helloMethod.visitInsn(Opcodes.ARETURN);
helloMethod.visitMaxs(3, 2);
helloMethod.visitEnd();
classWriter.visitEnd();
byte[] data = classWriter.toByteArray();
File file = new File("D://Hello.class");
FileOutputStream fileOutputStream = new FileOutputStream(file);
fileOutputStream.write(data);
fileOutputStream.close();
}
}
内容很简单，通过ClassWriter创建了一个Hello类，定义了一个构造方法
ASM设计模式很有意思，访问者模式（Visitor Pattern），我常常把它比作一个拥有多层的抽屉的模具，一层一层打开，放原材料，最后产出一个东西。MethodVisitor的用法可以参考文档，这里不详述，大致上都是一些字节码指令操作。
以上代码也是模仿了javap反编译出来的内容，运行后无误的话可以看到在D盘有了一个Hello.class文件，接下来我们要加载这个文件，并运行其中的方法。代码如下：
package classloader;
/**
* Created by dorole.com on 2016/6/13.
*/
public class MyLoader extends ClassLoader {
public Class<?> defineMyClass(byte[] b, int off, int len) {
return super.defineClass(null, b, off, len);
}
}
自定义一个Classloader，仅调用父类的defineClass即可。
package classloader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
/**
* Created by dorole.com on 2016/6/13.
*/
public class MyLoaderTest {
public static void main(String[] args) throws IOException, IllegalAccessException, InstantiationException {
File file = new File("D://Hello.class");
InputStream input = new FileInputStream(file);
byte[] result = new byte[1024];
int count = input.read(result);
MyLoader loader = new MyLoader();
Class clazz = loader.defineMyClass(result, 0, count);
System.out.println(clazz.getCanonicalName());
Object o = clazz.newInstance();
try {
Method method = clazz.getMethod("sayHello", String.class);
Object returnObject = method.invoke(o, "World!");
System.out.println("returnObject = " + returnObject);
} catch (IllegalArgumentException | InvocationTargetException
| NoSuchMethodException | SecurityException e) {
e.printStackTrace();
}
}
}
将class以字节数组形式读取给Classloader来加载，通过反射调用sayHello方法，专递一个World!参数进去，可以看到输出以下结果：
Hello Hello -> World! returnObject = World!
这样一个从Java代码编译与反编译，自己生成字节码到加载运行，还能反编译回去。这样一个完整的循环就算完成了，虽然谈不上很有深度，但从这里能引发很多值得思考的地方。更多内容还是要参考Java虚拟机规范。
