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Java中的cglib原理解析

作者：bluespacezero

这篇文章主要介绍了Java中的cglib原理解析,由于代理类继承了被代理类,所以调用sayHello()方法时会直接调用代理类的sayHello()方法,而在代理类的方法中,调用了Callback的逻辑,需要的朋友可以参考下

cglib原理解析

先放上示例代码：

//Person.java
public class Person {
    public void sayHello() {
        System.out.println("Hello!");
    }
}

//Test.java
import org.objectweb.asm.ClassWriter;
import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;

import java.io.FileOutputStream;
import java.lang.reflect.Method;

public class Test {
    public static void main(String[] args) throws Exception {
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(Person.class);
        //设置生成的类不继承Factory接口
        enhancer.setUseFactory(false);
        enhancer.setCallback(new MethodInterceptor() {
            public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
                System.out.println("Intercept!");
                methodProxy.invokeSuper(o, objects);
                return null;
            }
        });

        Person person = (Person) enhancer.create();
        person.sayHello();

        //以下代码是为了输出cglib生成的类的class文件
        ClassWriter cw = new ClassWriter(0);
        enhancer.generateClass(cw);
        byte[] klass = cw.toByteArray();
        FileOutputStream fileOutputStream = new FileOutputStream(person.getClass().getName()+".class");
        fileOutputStream.write(klass);
        fileOutputStream.close();
    }
}

使用cglib生成代码的过程很简单：

为了简单起见，我们还调用了setUseFactory(false)方法，使得生成的代理类不实现Factory接口（该接口提供了一些生成新对象的和设置对象的Callback的方法）。并且没有使用CallbackFilter。

cglib生成对象时主要的逻辑在Enchancer类的generateClass方法里面：

public void generateClass(ClassVisitor v) throws Exception {
        Class sc = (superclass == null) ? Object.class : superclass;

        //如果被代理类是final类型的，抛出异常
        if (TypeUtils.isFinal(sc.getModifiers()))
            throw new IllegalArgumentException("Cannot subclass final class " + sc.getName());
        //以下两行代码获得被代理类的非私有构造函数
        List constructors = new ArrayList(Arrays.asList(sc.getDeclaredConstructors()));
        filterConstructors(sc, constructors);

        // Order is very important: must add superclass, then
        // its superclass chain, then each interface and
        // its superinterfaces.
        List actualMethods = new ArrayList();
        List interfaceMethods = new ArrayList();
        final Set forcePublic = new HashSet();
        //得到被代理类及其父类、接口的非final、非static、非private方法。这些方法是需要代理的（在不考虑CallbackFilter的情况下）
        getMethods(sc, interfaces, actualMethods, interfaceMethods, forcePublic);

        //获得需要代理的方法的方法信息
        List methods = CollectionUtils.transform(actualMethods, new Transformer() {
            public Object transform(Object value) {
                Method method = (Method)value;
                int modifiers = Constants.ACC_FINAL
                    | (method.getModifiers()
                       & ~Constants.ACC_ABSTRACT
                       & ~Constants.ACC_NATIVE
                       & ~Constants.ACC_SYNCHRONIZED);
                if (forcePublic.contains(MethodWrapper.create(method))) {
                    modifiers = (modifiers & ~Constants.ACC_PROTECTED) | Constants.ACC_PUBLIC;
                }
                return ReflectUtils.getMethodInfo(method, modifiers);
            }
        });

        ClassEmitter e = new ClassEmitter(v);
        if (currentData == null) {
            //生成class文件的版本号、类的访问描述符、类名、父类名、接口，并生成SourceFile属性
            e.begin_class(Constants.V1_2,
                      Constants.ACC_PUBLIC,
                      getClassName(),
                      Type.getType(sc),
                      (useFactory ?
                       TypeUtils.add(TypeUtils.getTypes(interfaces), FACTORY) :
                       TypeUtils.getTypes(interfaces)),
                      Constants.SOURCE_FILE);
        } else {
            e.begin_class(Constants.V1_2,
                    Constants.ACC_PUBLIC,
                    getClassName(),
                    null,
                    new Type[]{FACTORY},
                    Constants.SOURCE_FILE);
        }
        List constructorInfo = CollectionUtils.transform(constructors, MethodInfoTransformer.getInstance());

        //以下生成代理类的字段，可以对照下面的反编译结果观看
        //生成private boolean CGLIB$BOUND字段
        e.declare_field(Constants.ACC_PRIVATE, BOUND_FIELD, Type.BOOLEAN_TYPE, null);
        //生成public static Object CGLIB$FACTORY_DATA字段
        e.declare_field(Constants.ACC_PUBLIC | Constants.ACC_STATIC, FACTORY_DATA_FIELD, OBJECT_TYPE, null);
        if (!interceptDuringConstruction) {
            e.declare_field(Constants.ACC_PRIVATE, CONSTRUCTED_FIELD, Type.BOOLEAN_TYPE, null);
        }
        //生成private static final ThreadLocal CGLIB$THREAD_CALLBACKS字段
        e.declare_field(Constants.PRIVATE_FINAL_STATIC, THREAD_CALLBACKS_FIELD, THREAD_LOCAL, null);
        //生成private static final Callback[] CGLIB$STATIC_CALLBACKS字段
        e.declare_field(Constants.PRIVATE_FINAL_STATIC, STATIC_CALLBACKS_FIELD, CALLBACK_ARRAY, null);
        if (serialVersionUID != null) {
            e.declare_field(Constants.PRIVATE_FINAL_STATIC, Constants.SUID_FIELD_NAME, Type.LONG_TYPE, serialVersionUID);
        }

        //以下是生成Callback字段，可以看到，对应每个设置的Callback，都会在代理类中生成一个对应具体类型的字段
        for (int i = 0; i < callbackTypes.length; i++) {
            //在本例中生成的是private MethodInterceptor CGLIB$CALLBACK_0字段，后缀_0代表是在Enhancer中设置的第0个Callback
            e.declare_field(Constants.ACC_PRIVATE, getCallbackField(i), callbackTypes[i], null);
        }
        // This is declared private to avoid "public field" pollution
        //生成private static Object CGLIB$CALLBACK_FILTER字段。如果Enhancer只设置了一个Callback并且没有设置CallbackFilter，那么会默认对该Enhancer对象添加一个指向第0个Callback的CallbackFilter
        e.declare_field(Constants.ACC_PRIVATE | Constants.ACC_STATIC, CALLBACK_FILTER_FIELD, OBJECT_TYPE, null);

        if (currentData == null) {
            //这个函数生成需要代理类的方法
            emitMethods(e, methods, actualMethods);
            //生成构造函数
            emitConstructors(e, constructorInfo);
        } else {
            emitDefaultConstructor(e);
        }
        //以下三行代码分别生成代理类的CGLIB$SET_THREAD_CALLBACKS、CGLIB$SET_STATIC_CALLBACKS和CGLIB$BIND_CALLBACKS函数
        emitSetThreadCallbacks(e);
        emitSetStaticCallbacks(e);
        emitBindCallbacks(e);

        if (useFactory || currentData != null) {
            int[] keys = getCallbackKeys();
            //如果useFactory为true就会生成几个newInstance方法，还有几个操作代理类的Callback的方法
            emitNewInstanceCallbacks(e);
            emitNewInstanceCallback(e);
            emitNewInstanceMultiarg(e, constructorInfo);
            emitGetCallback(e, keys);
            emitSetCallback(e, keys);
            emitGetCallbacks(e);
            emitSetCallbacks(e);
        }

        e.end_class();
    }

使用Bytecode Viewer的CFR反编译生成的代理类如下：

/*
 * Decompiled with CFR 0_125.
 * 
 * Could not load the following classes:
 *  Person
 *  Person$$EnhancerByCGLIB$$34b905e4
 *  net.sf.cglib.core.ReflectUtils
 *  net.sf.cglib.core.Signature
 *  net.sf.cglib.proxy.Callback
 *  net.sf.cglib.proxy.MethodInterceptor
 *  net.sf.cglib.proxy.MethodProxy
 */
import java.lang.reflect.Method;
import net.sf.cglib.core.ReflectUtils;
import net.sf.cglib.core.Signature;
import net.sf.cglib.proxy.Callback;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
/*
 * Exception performing whole class analysis ignored.
 */
public class Person$$EnhancerByCGLIB$$34b905e4
extends Person {
    private boolean CGLIB$BOUND;
    public static Object CGLIB$FACTORY_DATA;
    private static final ThreadLocal CGLIB$THREAD_CALLBACKS;
    private static final Callback[] CGLIB$STATIC_CALLBACKS;
    private MethodInterceptor CGLIB$CALLBACK_0;
    private static Object CGLIB$CALLBACK_FILTER;
    private static final Method CGLIB$sayHello$0$Method;
    private static final MethodProxy CGLIB$sayHello$0$Proxy;
    private static final Object[] CGLIB$emptyArgs;
    private static final Method CGLIB$equals$1$Method;
    private static final MethodProxy CGLIB$equals$1$Proxy;
    private static final Method CGLIB$toString$2$Method;
    private static final MethodProxy CGLIB$toString$2$Proxy;
    private static final Method CGLIB$hashCode$3$Method;
    private static final MethodProxy CGLIB$hashCode$3$Proxy;
    private static final Method CGLIB$clone$4$Method;
    private static final MethodProxy CGLIB$clone$4$Proxy;
    static void CGLIB$STATICHOOK2() {
        CGLIB$THREAD_CALLBACKS = new ThreadLocal();
        CGLIB$emptyArgs = new Object[0];
        Class class_ = Class.forName("Person$$EnhancerByCGLIB$$34b905e4");
        Class class_2 = Class.forName("java.lang.Object");
        Method[] arrmethod = ReflectUtils.findMethods((String[])new String[]{"equals", "(Ljava/lang/Object;)Z", "toString", "()Ljava/lang/String;", "hashCode", "()I", "clone", "()Ljava/lang/Object;"}, (Method[])class_2.getDeclaredMethods());
        CGLIB$equals$1$Method = arrmethod[0];
        CGLIB$equals$1$Proxy = MethodProxy.create(class_2, class_, (String)"(Ljava/lang/Object;)Z", (String)"equals", (String)"CGLIB$equals$1");
        CGLIB$toString$2$Method = arrmethod[1];
        CGLIB$toString$2$Proxy = MethodProxy.create(class_2, class_, (String)"()Ljava/lang/String;", (String)"toString", (String)"CGLIB$toString$2");
        CGLIB$hashCode$3$Method = arrmethod[2];
        CGLIB$hashCode$3$Proxy = MethodProxy.create(class_2, class_, (String)"()I", (String)"hashCode", (String)"CGLIB$hashCode$3");
        CGLIB$clone$4$Method = arrmethod[3];
        CGLIB$clone$4$Proxy = MethodProxy.create(class_2, class_, (String)"()Ljava/lang/Object;", (String)"clone", (String)"CGLIB$clone$4");
        class_2 = Class.forName("Person");
        CGLIB$sayHello$0$Method = ReflectUtils.findMethods((String[])new String[]{"sayHello", "()V"}, (Method[])class_2.getDeclaredMethods())[0];
        CGLIB$sayHello$0$Proxy = MethodProxy.create(class_2, class_, (String)"()V", (String)"sayHello", (String)"CGLIB$sayHello$0");
    }
    final void CGLIB$sayHello$0() {
        super.sayHello();
    }
    public final void sayHello() {
        MethodInterceptor methodInterceptor = this.CGLIB$CALLBACK_0;
        if (methodInterceptor == null) {
            Person$$EnhancerByCGLIB$$34b905e4.CGLIB$BIND_CALLBACKS((Object)this);
            methodInterceptor = this.CGLIB$CALLBACK_0;
        }
        if (methodInterceptor != null) {
            Object object = methodInterceptor.intercept((Object)this, CGLIB$sayHello$0$Method, CGLIB$emptyArgs, CGLIB$sayHello$0$Proxy);
            return;
        }
        super.sayHello();
    }
    final boolean CGLIB$equals$1(Object object) {
        return super.equals(object);
    }
    public final boolean equals(Object object) {
        MethodInterceptor methodInterceptor = this.CGLIB$CALLBACK_0;
        if (methodInterceptor == null) {
            Person$$EnhancerByCGLIB$$34b905e4.CGLIB$BIND_CALLBACKS((Object)this);
            methodInterceptor = this.CGLIB$CALLBACK_0;
        }
        if (methodInterceptor == null) return super.equals(object);
        Object object2 = methodInterceptor.intercept((Object)this, CGLIB$equals$1$Method, new Object[]{object}, CGLIB$equals$1$Proxy);
        if (object2 == null) {
            return false;
        }
        boolean bl = (Boolean)object2;
        return bl;
    }
    final String CGLIB$toString$2() {
        return super.toString();
    }
    public final String toString() {
        MethodInterceptor methodInterceptor = this.CGLIB$CALLBACK_0;
        if (methodInterceptor == null) {
            Person$$EnhancerByCGLIB$$34b905e4.CGLIB$BIND_CALLBACKS((Object)this);
            methodInterceptor = this.CGLIB$CALLBACK_0;
        }
        if (methodInterceptor == null) return super.toString();
        return (String)methodInterceptor.intercept((Object)this, CGLIB$toString$2$Method, CGLIB$emptyArgs, CGLIB$toString$2$Proxy);
    }
    final int CGLIB$hashCode$3() {
        return super.hashCode();
    }
    public final int hashCode() {
        MethodInterceptor methodInterceptor = this.CGLIB$CALLBACK_0;
        if (methodInterceptor == null) {
            Person$$EnhancerByCGLIB$$34b905e4.CGLIB$BIND_CALLBACKS((Object)this);
            methodInterceptor = this.CGLIB$CALLBACK_0;
        }
        if (methodInterceptor == null) return super.hashCode();
        Object object = methodInterceptor.intercept((Object)this, CGLIB$hashCode$3$Method, CGLIB$emptyArgs, CGLIB$hashCode$3$Proxy);
        if (object == null) {
            return 0;
        }
        int n = ((Number)object).intValue();
        return n;
    }
    final Object CGLIB$clone$4() throws CloneNotSupportedException {
        return super.clone();
    }
    protected final Object clone() throws CloneNotSupportedException {
        MethodInterceptor methodInterceptor = this.CGLIB$CALLBACK_0;
        if (methodInterceptor == null) {
            Person$$EnhancerByCGLIB$$34b905e4.CGLIB$BIND_CALLBACKS((Object)this);
            methodInterceptor = this.CGLIB$CALLBACK_0;
        }
        if (methodInterceptor == null) return super.clone();
        return methodInterceptor.intercept((Object)this, CGLIB$clone$4$Method, CGLIB$emptyArgs, CGLIB$clone$4$Proxy);
    }
    public static MethodProxy CGLIB$findMethodProxy(Signature signature) {
        String string = signature.toString();
        switch (string.hashCode()) {
            case -508378822: {
                if (!string.equals("clone()Ljava/lang/Object;")) return null;
                return CGLIB$clone$4$Proxy;
            }
            case 1535311470: {
                if (!string.equals("sayHello()V")) return null;
                return CGLIB$sayHello$0$Proxy;
            }
            case 1826985398: {
                if (!string.equals("equals(Ljava/lang/Object;)Z")) return null;
                return CGLIB$equals$1$Proxy;
            }
            case 1913648695: {
                if (!string.equals("toString()Ljava/lang/String;")) return null;
                return CGLIB$toString$2$Proxy;
            }
            case 1984935277: {
                if (!string.equals("hashCode()I")) return null;
                return CGLIB$hashCode$3$Proxy;
            }
        }
        return null;
    }
    public Person$$EnhancerByCGLIB$$34b905e4() {
        Person$$EnhancerByCGLIB$$34b905e4 person$$EnhancerByCGLIB$$34b905e4 = this;
        Person$$EnhancerByCGLIB$$34b905e4.CGLIB$BIND_CALLBACKS((Object)person$$EnhancerByCGLIB$$34b905e4);
    }
    public static void CGLIB$SET_THREAD_CALLBACKS(Callback[] arrcallback) {
        CGLIB$THREAD_CALLBACKS.set(arrcallback);
    }
    public static void CGLIB$SET_STATIC_CALLBACKS(Callback[] arrcallback) {
        CGLIB$STATIC_CALLBACKS = arrcallback;
    }
    private static final void CGLIB$BIND_CALLBACKS(Object object) {
        Person$$EnhancerByCGLIB$$34b905e4 person$$EnhancerByCGLIB$$34b905e4 = (Person$$EnhancerByCGLIB$$34b905e4)object;
        if (person$$EnhancerByCGLIB$$34b905e4.CGLIB$BOUND) return;
        person$$EnhancerByCGLIB$$34b905e4.CGLIB$BOUND = true;
        Object t = CGLIB$THREAD_CALLBACKS.get();
        if (t == null && (v783 = CGLIB$STATIC_CALLBACKS) == null) {
            return;
        }
        person$$EnhancerByCGLIB$$34b905e4.CGLIB$CALLBACK_0 = (MethodInterceptor)((Callback[])t)[0];
    }
    static {
        Person$$EnhancerByCGLIB$$34b905e4.CGLIB$STATICHOOK2();
    }
}

可以看到，在创建一个代理类的对象的时候，会先加载这个类，也就调用了类的静态方法CGLIB$STATICHOOK2()，在这个函数中对类的其它静态字段进行了初始化，比如对应每个需要代理的方法都生成两个字段，一个是用于反射的Method对象，另一个是用于加快方法调用、避免反射带来的开销的MethodProxy对象。然后在构造函数里面调用了CGLIB$BIND_CALLBACKS(this)，在CGLIB$BIND_CALLBACKS方法中会把private MethodInterceptor CGLIB$CALLBACK_0字段和设置的静态变量ThreadLocal CGLIB$THREAD_CALLBACKS进行绑定，也就是说，设置的Callback并不是硬编码在生成的代理类里面的，而是可以动态地设置。

由于代理类继承了被代理类，所以调用sayHello()方法时会直接调用代理类的sayHello()方法，而在代理类的方法中，调用了Callback的逻辑。

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