Jdk动态代理 底层源码

2021年1月16日 8点热度 0条评论 来源: bugpool

Spring AOP源码目录

Spring AOP总览:灵魂画手图解Spring AOP实现原理
Spring AOP源码01:Jdk动态代理底层源码
Spring AOP源码02:ProxyFactory
Spring AOP源码03:JdkDynamicAopProxy
Spring AOP源码04:MethodInvocation 拦截器调用
Spring AOP源码05:DefaultAdvisorAutoProxyCreator
Spring期末考压轴题:当Spring AOP遇上循环依赖
git注释源码地址:https://github.com/chaitou/spring-framework-master.git

前言

java动态代理主要有2种,Jdk动态代理、Cglib动态代理,本文主要讲解Jdk动态代理的使用、运行机制、以及源码分析。当spring没有手动开启Cglib动态代理,即:<aop:aspectj-autoproxy proxy-target-class="true"/>@EnableAspectJAutoProxy(proxyTargetClass = true),默认使用的就是Jdk动态代理。动态代理的应用范围很广,例如:日志、事务管理、缓存等。本文将模拟@Cacheable,即缓存在动态代理中的应用进行讲解。需要注意的是,Jdk动态代理相比起cglib动态代理,Jdk动态代理的对象必须实现接口,否则将报错。我们也将带着这个问题在源码分析中寻找答案

当@Cacheable注解在方法上时

  1. 在方法执行前,将调用Jdk动态代理优先查找Redis(或其他缓存)
  2. 当缓存不存在时,执行方法,例如查询数据库
  3. 在方法执行后,再次调用Jdk动态代理,将结果缓存到Redis中

一、使用

步骤

  1. 创建接口UserService
  2. 创建接口实现类UserServiceImpl
  3. 创建Jdk动态代理JdkCacheHandler,用于增强UserServiceImpl方法前后的缓存逻辑

代码

  1. 创建接口UserService
public interface UserService { 
    public String getUserByName(String name);
}
  1. 创建实现类UserServiceImpl
public class UserServiceImpl implements UserService { 
    @Override
    public String getUserByName(String name) { 
        System.out.println("从数据库中查询到:" + name);
        return name;
    }
}
  1. 创建Jdk动态代理JdkCacheHandler
public class JdkCacheHandler implements InvocationHandler { 

    // 目标类对象
    private Object target;

    // 获取目标类对象
    public JdkCacheHandler(Object target) { 
        this.target = target;
    }

    // 创建JDK代理
    public Object createJDKProxy() { 
        Class clazz = target.getClass();
        // 创建JDK代理需要3个参数,目标类加载器、目标类接口、代理类对象(即本身)
        return Proxy.newProxyInstance(clazz.getClassLoader(), clazz.getInterfaces(), this);
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 

        System.out.println("查找数据库前,在缓存中查找是否存在:" + args[0]);
        // 触发目标类方法
        Object result = method.invoke(target, args);
        System.out.printf("查找数据库后,将%s加入到缓存中\r\n", result);
        return result;
    }
}
  1. 创建测试类
public class JdkTest { 

    @Test
    public void test() { 
        UserService userService = new UserServiceImpl();
        JdkCacheHandler jdkCacheHandler = new JdkCacheHandler(userService);
        UserService proxy = (UserService) jdkCacheHandler.createJDKProxy();

        System.out.println("==========================");
        proxy.getUserByName("bugpool");
        System.out.println("==========================");

        System.out.println(proxy.getClass());
    }
}
  1. 输出
==========================
查找数据库前,在缓存中查找是否存在:bugpool
从数据库中查询到:bugpool
查找数据库后,将bugpool加入到缓存中
==========================
class com.sun.proxy.$Proxy4

二、调用机制

查看$Proxy代码

可以看到当经过Jdk动态代理以后,生产的proxy已经不再是UserService类型了,而是$Proxy4类型,想要了解其调用机制,得先获取到proxy类的代码

System.out.println(proxy.getClass());

class com.sun.proxy.$Proxy4
  1. 修改JVM运行参数,添加-Dsun.misc.ProxyGenerator.saveGeneratedFiles=true

  2. 运行test即可在com.sun.proxy查看代码,此时生产的是class,idea打开会自动反编译
  3. 在上方输出中可以看到代理类是$Proxy4,至此获取到$Proxy4的源代码,接下去分析代理类的调用机制
public final class $Proxy4 extends Proxy implements UserService { 
    private static Method m1;
    private static Method m3;
    private static Method m2;
    private static Method m0;

    public $Proxy4(InvocationHandler var1) throws  { 
        super(var1);
    }

    public final boolean equals(Object var1) throws  { 
        try { 
            return (Boolean)super.h.invoke(this, m1, new Object[]{ var1});
        } catch (RuntimeException | Error var3) { 
            throw var3;
        } catch (Throwable var4) { 
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final String getUserByName(String var1) throws  { 
        try { 
            return (String)super.h.invoke(this, m3, new Object[]{ var1});
        } catch (RuntimeException | Error var3) { 
            throw var3;
        } catch (Throwable var4) { 
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final String toString() throws  { 
        try { 
            return (String)super.h.invoke(this, m2, (Object[])null);
        } catch (RuntimeException | Error var2) { 
            throw var2;
        } catch (Throwable var3) { 
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final int hashCode() throws  { 
        try { 
            return (Integer)super.h.invoke(this, m0, (Object[])null);
        } catch (RuntimeException | Error var2) { 
            throw var2;
        } catch (Throwable var3) { 
            throw new UndeclaredThrowableException(var3);
        }
    }

    static { 
        try { 
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m3 = Class.forName("proxy.jdk.UserService").getMethod("getUserByName", Class.forName("java.lang.String"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) { 
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) { 
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

调用机制

  1. 从proxy调用开始
// JdkTest.java
proxy.getUserByName("bugpool");
  1. proxy是$Proxy4类型,因此进入$Proxy4的getUserByName方法
// $Proxy4.class
public final class $Proxy4 extends Proxy implements UserService { 
    ...

    // 构造器,传入JdkCacheHandler类的对象,正是下方调用的super.h属性
    public $Proxy4(InvocationHandler var1) throws  { 
        super(var1);
    }

    public final String getUserByName(String var1) throws  { 
        try { 
            /** * 调用父类的h属性的invoke方法 * 在下面的源码分析中,会发现h属性正是JdkCacheHandler类createJDKProxy方法中所传入的this * Proxy.newProxyInstance(clazz.getClassLoader(), clazz.getInterfaces(), this); * 而this指代的正是JdkCacheHandler类的对象,因此最后调用的是JdkCacheHandler的invoke方法 */
            return (String)super.h.invoke(this, m3, new Object[]{ var1});
        } catch (RuntimeException | Error var3) { 
            throw var3;
        } catch (Throwable var4) { 
            throw new UndeclaredThrowableException(var4);
        }
    }

    ...

    static { 
        ...
        m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
        m3 = Class.forName("proxy.jdk.UserService").getMethod("getUserByName", Class.forName("java.lang.String"));
        m2 = Class.forName("java.lang.Object").getMethod("toString");
        m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        ...
    }
}
  1. 因此h.invok实际调用的正是JdkCacheHandler类的invoke方法
// JdkCacheHandler.java
public class JdkCacheHandler implements InvocationHandler { 
    ...
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 

        System.out.println("查找数据库前,在缓存中查找是否存在:" + args[0]);
        // 触发目标类方法
        Object result = method.invoke(target, args);
        System.out.printf("查找数据库后,将%s加入到缓存中\r\n", result);
        return result;
    }
}
  1. method.invoke(target, args)中的method = getUserByName,target = 构造函数传进来的UserServiceImpl对象,args = “bugpool”
// UserServiceImpl.java
public class UserServiceImpl implements UserService { 
    @Override
    public String getUserByName(String name) { 
        System.out.println("从数据库中查询到:" + name);
        return name;
    }
}

三、源码分析

原理

了解完Jdk动态代理的调用机制,所有核心问题都落在了$Proxy4类的对象proxy是如何生成的上面?即下面这句代码上,这里先给出概述,有利于宏观上看源码。在开始之前先复习一下java的运行机制:1. 所有.java文件经过编译生成.class文件 2. 通过类加载器classLoad将.class中的字节码加载到JVM中 3. 运行

// 创建JDK代理
public Object createJDKProxy() { 
    Class clazz = target.getClass();
    // 创建JDK代理需要3个参数
    // 目标类加载器:用于加载生成的字节码
    // 目标类接口:用于生成字节码,也就是说$Proxy的生产仅仅需要接口数组就可以完成
    // 代理类对象(即本身):用于回调invoke方法,实现方法的增强
    return Proxy.newProxyInstance(clazz.getClassLoader(), clazz.getInterfaces(), this);
}
  1. 通过clazz.getInterfaces()获取到所有接口,通过接口可以生成类似以下字节码(注意以下给出的是代码),细细观察会发现其实各个接口方法生成的代码都是一样的,只有(String)super.h.invoke(this, m3, new Object[]{var1}的m3和参数有可能是不同的。所以其实想生成$Proxy字节码,只需要接口数组就已经完全足够了
public final String getUserByName(String var1) throws  { 
        try { 
            return (String)super.h.invoke(this, m3, new Object[]{ var1});
        } catch (RuntimeException | Error var3) { 
            throw var3;
        } catch (Throwable var4) { 
            throw new UndeclaredThrowableException(var4);
        }
    }
  1. 此时已经获取到$Proxy4.class的字节码,但是此处的字节码还未加载到JVM中,因此需要调用clazz.getClassLoader()传进来的类加载器进行加载,并得到对应的class,也就是$Proxy类
  2. 获取$Proxy类的构造函数,该构造函数有一个重要的参数h
  3. 通过反射调用$Proxy类的构造函数,cons.newInstance(new Object[]{h});构造函数的h正是传入的this,也就是JdkCacheHandler类的对象
  4. 将反射获取到的$Proxy对象放回

源码分析

  1. Proxy.newProxyInstance开始跟踪代码(注:①代表上方概述的步骤1)
// JdkCacheHandler.java
// 创建JDK代理
public Object createJDKProxy() { 
    Class clazz = target.getClass();
    // 创建JDK代理需要3个参数,目标类加载器、目标类接口、代理类对象(即本身)
    return Proxy.newProxyInstance(clazz.getClassLoader(), clazz.getInterfaces(), this);
}
  1. 跟踪代码newProxyInstance,这里需要注意在①②过程结束后,③④过程调用前,即Class<?> cl = getProxyClass0(loader, intfs);结束后,cl变量一直都只是class,即$Proxy4类,并未生成对应的对象,这里不要混淆类和对象
// Proxy.java
// loader类加载器,interfaces目标类实现的所有接口,h即InvocationHandler类的对象
public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    { 
        // 校验InvocationHandler是否为空
        Objects.requireNonNull(h);

        // 该目标类实现的接口数组
        final Class<?>[] intfs = interfaces.clone();
        // 安全检查
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) { 
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        /* * Look up or generate the designated proxy class. */
        // 当缓存中存在代理类则直接获取,否则生成代理类
        // ①②步骤,核心代码,即生成代理类字节码以及加载都在这里进行
        Class<?> cl = getProxyClass0(loader, intfs);

        /* * Invoke its constructor with the designated invocation handler. */
        try { 
            if (sm != null) { 
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }

            // ③ 从生成的代理类中获取构造函数
            // constructorParams = { InvocationHandler.class };
            final Constructor<?> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) { 
                AccessController.doPrivileged(new PrivilegedAction<Void>() { 
                    public Void run() { 
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            // ④ 调用构造函数,将InvocationHandler作为参数实例化代理对象
            return cons.newInstance(new Object[]{ h});
        } catch (IllegalAccessException|InstantiationException e) { 
            throw new InternalError(e.toString(), e);
        } catch (InvocationTargetException e) { 
            Throwable t = e.getCause();
            if (t instanceof RuntimeException) { 
                throw (RuntimeException) t;
            } else { 
                throw new InternalError(t.toString(), t);
            }
        } catch (NoSuchMethodException e) { 
            throw new InternalError(e.toString(), e);
        }
    }
  1. 跟踪Class<?> cl = getProxyClass0(loader, intfs);
// Proxy.java
private static Class<?> getProxyClass0(ClassLoader loader,
                                       Class<?>... interfaces) { 
    if (interfaces.length > 65535) { 
        throw new IllegalArgumentException("interface limit exceeded");
    }

    // If the proxy class defined by the given loader implementing
    // the given interfaces exists, this will simply return the cached copy;
    // otherwise, it will create the proxy class via the ProxyClassFactory
    // 如果在类加载器中已经存在实现了对应接口的代理类,则直接返回缓存中的代理类
    // 否则,通过ProxyClassFactory新建代理类
    return proxyClassCache.get(loader, interfaces);
}
  1. 跟踪proxyClassCache.get(loader, interfaces);(注:Jdk动态代理对已经生成加载过的代理类进行了缓存以提高性能,缓存的相关代码不是我们关心的重点,可以跳过相关代码)本段代码我们主要关心V value = supplier.get();其中supplier本质是factory,通过new Factory(key, parameter, subKey, valuesMap)创建
// WeakCache.java
//K和P就是WeakCache定义中的泛型,key是类加载器,parameter是接口类数组
public V get(K key, P parameter) { 
        // 检查接口数组是否为空
        Objects.requireNonNull(parameter);

        expungeStaleEntries();

        Object cacheKey = CacheKey.valueOf(key, refQueue);

        // lazily install the 2nd level valuesMap for the particular cacheKey
        ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
        if (valuesMap == null) { 
            ConcurrentMap<Object, Supplier<V>> oldValuesMap
                = map.putIfAbsent(cacheKey,
                                  valuesMap = new ConcurrentHashMap<>());
            if (oldValuesMap != null) { 
                valuesMap = oldValuesMap;
            }
        }

        // create subKey and retrieve the possible Supplier<V> stored by that
        // subKey from valuesMap
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        //通过sub-key得到supplier,实质就是factory
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) { 
            if (supplier != null) { 
                // supplier might be a Factory or a CacheValue<V> instance
                // ①②步骤都在这里,如果supplier不为空,则直接调用get方法返回代理类
                V value = supplier.get();
                if (value != null) { 
                    return value;
                }
            }
            // else no supplier in cache
            // or a supplier that returned null (could be a cleared CacheValue
            // or a Factory that wasn't successful in installing the CacheValue)

            // lazily construct a Factory
            if (factory == null) { 
                // 创建对应factory,此段代码在死循环中,下一次supplier.get()将会获取到代理类并退出循环
                factory = new Factory(key, parameter, subKey, valuesMap);
            }

            if (supplier == null) { 
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) { 
                    // successfully installed Factory
                    // 赋值给supplier
                    supplier = factory;
                }
                // else retry with winning supplier
            } else { 
                if (valuesMap.replace(subKey, supplier, factory)) { 
                    // successfully replaced
                    // cleared CacheEntry / unsuccessful Factory
                    // with our Factory
                    supplier = factory;
                } else { 
                    // retry with current supplier
                    supplier = valuesMap.get(subKey);
                }
            }
        }
    }
  1. 跟踪V value = supplier.get();即Factory类的get方法,这里大部分的工作还是在做校验和缓存,我们只关心核心逻辑valueFactory.apply(key, parameter);其中valueFactory是上一个步骤传入的ProxyClassFactory
// Factory.java 
public synchronized V get() {  // serialize access
        // re-check
        // 再次检查,supplier是否是当前对象
        Supplier<V> supplier = valuesMap.get(subKey);
        if (supplier != this) { 
            // something changed while we were waiting:
            // might be that we were replaced by a CacheValue
            // or were removed because of failure ->
            // return null to signal WeakCache.get() to retry
            // the loop
            return null;
        }
        // else still us (supplier == this)

        // create new value
        V value = null;
        try { 
            // valueFactory 是前序传进来的 new ProxyClassFactory()
            // ①②步骤,核心逻辑,调用valueFactory.apply生成对应代理类并加载
            value = Objects.requireNonNull(valueFactory.apply(key, parameter));
        } finally { 
            if (value == null) {  // remove us on failure
                valuesMap.remove(subKey, this);
            }
        }
        // the only path to reach here is with non-null value
        assert value != null;

        // wrap value with CacheValue (WeakReference)
        CacheValue<V> cacheValue = new CacheValue<>(value);

        // put into reverseMap
        reverseMap.put(cacheValue, Boolean.TRUE);

        // try replacing us with CacheValue (this should always succeed)
        if (!valuesMap.replace(subKey, this, cacheValue)) { 
            throw new AssertionError("Should not reach here");
        }

        // successfully replaced us with new CacheValue -> return the value
        // wrapped by it
        return value;
    }
  1. 跟踪核心逻辑
    • Jdk动态代理通过拼凑的方式拼凑出$Proxy的全类名:com.sun.proxy.$proxy0.class
    • ③生产字节码byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags);可以看出Jdk动态代理需要interfaces接口数组进行生成字节码,这也是文章开头提出为什么必须实现接口的原因。同时从参数也可以看出需要生成字节码其实只需要接口数组,不需要其他信息。其实实现原理大概也可以猜出,Jdk动态代理通过遍历所有接口方法,为方法生成对应的return (String)super.h.invoke(this, m0~n, new Object[]{var1});代码
    • ④加载字节码:在③中获取到了字节码的字节数组,接下去就是调用classLoader将所有的字节码读入到JVM中
// ProxyClassFactory.java
private static final class ProxyClassFactory
        implements BiFunction<ClassLoader, Class<?>[], Class<?>>
    { 
        // prefix for all proxy class names
        // 代理类名称前缀
        private static final String proxyClassNamePrefix = "$Proxy";

        // next number to use for generation of unique proxy class names
        // 代理类计数器
        private static final AtomicLong nextUniqueNumber = new AtomicLong();

        @Override
        public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) { 

            Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            // 校验代理类接口
            for (Class<?> intf : interfaces) { 
                /* * Verify that the class loader resolves the name of this * interface to the same Class object. */
                Class<?> interfaceClass = null;
                try { 
                    interfaceClass = Class.forName(intf.getName(), false, loader);
                } catch (ClassNotFoundException e) { 
                }
                if (interfaceClass != intf) { 
                    throw new IllegalArgumentException(
                        intf + " is not visible from class loader");
                }
                /* * Verify that the Class object actually represents an * interface. */
                if (!interfaceClass.isInterface()) { 
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
                /* * Verify that this interface is not a duplicate. */
                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) { 
                    throw new IllegalArgumentException(
                        "repeated interface: " + interfaceClass.getName());
                }
            }

            // 代理类包名
            String proxyPkg = null;     // package to define proxy class in
            int accessFlags = Modifier.PUBLIC | Modifier.FINAL;

            /* * Record the package of a non-public proxy interface so that the * proxy class will be defined in the same package. Verify that * all non-public proxy interfaces are in the same package. */
            // 当接口修饰符是public,则所有包都可以使用
            // 当接口是非public,则生成的代理类必须和接口在与非public接口同一个包下
            // 如果非public的接口均在同一个包下,则生成的代理类放在非public接口同一个包下
            // 而如果非public的接口存在多个,且在不同包下,则抛出异常
            for (Class<?> intf : interfaces) { 
                int flags = intf.getModifiers();
                if (!Modifier.isPublic(flags)) { 
                    accessFlags = Modifier.FINAL;
                    String name = intf.getName();
                    int n = name.lastIndexOf('.');
                    String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
                    if (proxyPkg == null) { 
                        proxyPkg = pkg;
                    } else if (!pkg.equals(proxyPkg)) { 
                        throw new IllegalArgumentException(
                            "non-public interfaces from different packages");
                    }
                }
            }

            if (proxyPkg == null) { 
                // if no non-public proxy interfaces, use com.sun.proxy package
                // 如果都是公有的接口,则代理类默认放在com.sun.proxy package
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }

            /* * Choose a name for the proxy class to generate. */
            // 生成计数器,例如$proxy0~n
            long num = nextUniqueNumber.getAndIncrement();
            // 代理类名,com.sun.proxy.$proxy0.class
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /* * Generate the specified proxy class. */
            // ③生成代理类字节码
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            try { 
                // ④使用传进来的classLoader将代理类字节码加载到JVM中
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) { 
                /* * A ClassFormatError here means that (barring bugs in the * proxy class generation code) there was some other * invalid aspect of the arguments supplied to the proxy * class creation (such as virtual machine limitations * exceeded). */
                throw new IllegalArgumentException(e.toString());
            }
        }
    }
    原文作者:bugpool
    原文地址: https://blog.csdn.net/chaitoudaren/article/details/104833279
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