JDK1.8动态代理源码分析

最近在学习MyBatis源码时，想要查看下JDK是如何自动生成的Mapper代理类。于是仔细看了源码，在这里做个记录。

package com.br.itwzhangzx02.learn;

import learn.User;
import learn.UserMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.InputStream;

public class GeneratorClassFileTest {

  public static void main(String[] args) {
    //将生成的代理类class文件保存在磁盘
    System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
    String resource = "resources/mybatis-config.xml";
    SqlSessionFactory sqlSessionFactory;
    try (InputStream inputStream = Resources.getResourceAsStream(resource)) {
      //1、创建SqlSessionFactory
      sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
      //2、获取sqlSession
      SqlSession sqlSession = sqlSessionFactory.openSession();
      User user = getUser(sqlSession);
      System.out.println(user);
    }catch (Exception e){
    }
  }

  private static User getUser(SqlSession sqlSession) {
    //3、获取mapper，断点在这儿，然后进入
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
    userMapper.toString();
    //4、执行数据库操作，并处理结果集
    return userMapper.selectUser("10");
  }
  
}

 //MapperProxyFactory 类中的方法 第三个入参mapperProxy就是实现了InvocationHandler接口的类的对象
protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }



//这个是MapperRegistry类中的方法，将mapperProxyFactory 在初始化解析xml时缓存到内存中。
 public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {//这儿，用对应的工厂负责new一个代理类的对象
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

然后直接查看我们JDK的Proxy.newProxyInstance方法

 @CallerSensitive
    public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        Objects.requireNonNull(h);

        final Class<?>[] intfs = interfaces.clone();
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        /* 第一步：
         * Look up or generate the designated proxy class.
         * 这里使用到缓存技术，从缓存中取代理类，或者直接生成一个代理类
         */
        Class<?> cl = getProxyClass0(loader, intfs);

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

            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;
                    }
                });
            }
            //通过反射机制，拿到代理类的构造器对象，然后构造器对象创建代理类的实例
            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);
        }
    }


   private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }
        //第二步：
        // 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
       //注释很明白，就是先看缓存有没有，没有则创建代理类
        return proxyClassCache.get(loader, interfaces);
    }

//proxyClassCache 这个对象是什么？下面给proxyClassCache的赋值操作方法，
//final和static修饰的变量  ，所以在加载Proxy类的时候给proxyClassCache 赋值
 private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

//WeakCache类的构造方法 
//keyFactory是用来生产key的，ProxyClassFactory是用来生产代理类对象的，它俩都是实现了
//BiFunction接口的Proxy的内部类
 public WeakCache(BiFunction<K, P, ?> subKeyFactory,
                     BiFunction<K, P, V> valueFactory) {
        this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
        this.valueFactory = Objects.requireNonNull(valueFactory);
    }

//需要到WeakCache类中，找get方法

下面是WeakCache类的get方法

public V get(K key, P parameter) {
        Objects.requireNonNull(parameter);

        expungeStaleEntries();
        //对 key进行处理，将 null转换为一个 Object对象
        Object cacheKey = CacheKey.valueOf(key, refQueue);

        // lazily install the 2nd level valuesMap for the particular cacheKey
       //延迟初始化，二级MAP
        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
       //subKey的生成，需要查看Proxy中的KeyFactory内部类的apply方法，代码在下面
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
       //从我们的二级map中根据key，取出来的值可能是
       //代理类的工厂Factory 或缓存CacheValue
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
            if (supplier != null) {//缓存中有supplier
                // supplier might be a Factory or a CacheValue<V> instance
                V value = supplier.get();
                 //这儿取值可能情况有俩种，如果是Factory，则是生成代理类，可能失败
                //如果是CacheValue中，取值，可能为null
                if (value != null) {//supplier的get返回值不为空            
                    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) {
                factory = new Factory(key, parameter, subKey, valuesMap);
            }

            if (supplier == null) {
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) {
                    // successfully installed Factory
                    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);
                }
            }
        }
    }

//下面的代码是CacheKey中的
   private static final Object NULL_KEY = new Object();
   //对key进行包装，为NULL则返回一个Object对象
   static <K> Object valueOf(K key, ReferenceQueue<K> refQueue) {
            return key == null
                   // null key means we can't weakly reference it,
                   // so we use a NULL_KEY singleton as cache key
                   ? NULL_KEY
                   // non-null key requires wrapping with a WeakReference
                   : new CacheKey<>(key, refQueue);
        }

//WeakCache类中的map的定义，是一个二级MAP结构，同时用的是并发容器MAP
//简单理解，外层Map的key就是类加载器运算后生成的，里层的二级map的key就是类加载器和接口数组经
//过运算生成的，subMap中的value就是我们的代理类的一个持有者，factory或者cacheValue
    // the key type is Object for supporting null key
    private final ConcurrentMap<Object, ConcurrentMap<Object, Supplier<V>>> map
        = new ConcurrentHashMap<>();

//KeyFactory的定义，负责生成我们的subKey
private static final class KeyFactory
        implements BiFunction<ClassLoader, Class<?>[], Object>
    {
        @Override
        public Object apply(ClassLoader classLoader, Class<?>[] interfaces) {
            switch (interfaces.length) {
                case 1: return new Key1(interfaces[0]); // the most frequent
                case 2: return new Key2(interfaces[0], interfaces[1]);
                case 0: return key0;
                default: return new KeyX(interfaces);
            }
        }
    }

 上面分析：

V value = supplier.get();

这一步，我们只分析如果这个supplier是一个Factory时，也就是第一次执行UserMapper userMapper = sqlSession.getMapper(UserMapper.class);时如何生成代理类的对象（不是代理类的实例，是代表这个代理类的对象，具体可以查看java反射中Class）。第1次之后再调用时，我们的代理类对象从缓存ValueCache中拿，然后代理类对象拿到构造器对象，然后创建一个代理类的实例出来。（这个实例是，每次调用生成一个新的）

更简单的理解：生成的代理类Class只生成一次，然后缓存起来，之后每次调用getMapper时，新生成这个代理类的实例对象。

//Factory中的get方法
        @Override
        public synchronized V get() { // serialize access
            // re-check
            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
                //这一句判断很关键，比如我们的下单逻辑，只能执行一次，执行完之后状态修改，
               //别的线程，拿到锁之后，先查看状态，是否符合预期，如果状态不符合，
               //就不执行后续操作了。
               //或者一些要求幂等性的方法
                return null;
            }
            // else still us (supplier == this)

            // create new value
            V value = null;
            try {//创建我们的代理类对象
                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;
            //下面的逻辑，就是将我们的代理类对象缓存起来，将之前的subMap中的value值由factory
           //替换为我们的cacheValue ，
           //之后我们的获取代理类对象操作就不需要重新生成了，可以直接从缓存CacheValue 中get（）
            // wrap value with CacheValue (WeakReference)
            CacheValue<V> cacheValue = new CacheValue<>(value);

            // try replacing us with CacheValue (this should always succeed)
            if (valuesMap.replace(subKey, this, cacheValue)) {
                // put also in reverseMap
                reverseMap.put(cacheValue, Boolean.TRUE);
            } else {
                throw new AssertionError("Should not reach here");
            }

            // successfully replaced us with new CacheValue -> return the value
            // wrapped by it
            return value;
        }
    }

下面分析：factory生成代理类对象。前面说过这个factory是一个ProxyClassFactory类型。所以我们查看ProxyClassFactory里的apply方法。

value = Objects.requireNonNull(valueFactory.apply(key, parameter));

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

            Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            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());
                }
            }

            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.
             */
            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
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }

            /*
             * Choose a name for the proxy class to generate.
             */
            long num = nextUniqueNumber.getAndIncrement();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /*
             * Generate the specified proxy class.
             *关键代码，生成代理类的class文件，返回的是char数组
             */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            try {//返回代理类的对象
                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());
            }
        }
    }

private static final boolean saveGeneratedFiles = (Boolean)AccessController.doPrivileged(new GetBooleanAction("sun.misc.ProxyGenerator.saveGeneratedFiles"));

public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
        ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
        final byte[] var4 = var3.generateClassFile();
        if (saveGeneratedFiles) {//这个标识就是，控制是否将代理类class文件保存到磁盘
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    try {
                        int var1 = var0.lastIndexOf(46);
                        Path var2;
                        if (var1 > 0) {
                            Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar));
                            Files.createDirectories(var3);
                            var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
                        } else {
                            var2 = Paths.get(var0 + ".class");
                        }

                        Files.write(var2, var4, new OpenOption[0]);
                        return null;
                    } catch (IOException var4x) {
                        throw new InternalError("I/O exception saving generated file: " + var4x);
                    }
                }
            });
        }

        return var4;
    }

看代码知道，有个开关可以控制生成的代理类Class文件保存到磁盘中。

上面是我保存到本地的代理类的Class文件，下面是代码（反编译后的）

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package com.sun.proxy;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import learn.User;
import learn.UserMapper;

public final class $Proxy0 extends Proxy implements UserMapper {
    private static Method m1;
    private static Method m2;
    private static Method m3;
    private static Method m0;

    public $Proxy0(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 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 User selectUser(String var1) throws  {
        try {
            return (User)super.h.invoke(this, m3, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    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"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("learn.UserMapper").getMethod("selectUser", Class.forName("java.lang.String"));
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

总结：

1.我们多次调用

UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

实际上使用的同一个代理类，只有第一次是通过factory生成的，之后是从缓存ValueCache中取值。

2.我们生成的代理类是继承了Proxy类的，所以JDK动态代理不支持继承方式，只支持接口方式。

3.我们的InvocationHandler接口的实现类的对象h，作为构造函数的入参，传给我们的代理类。

4.我们代理类的代理方法是委托给h的invoke方法的。