代理模式

参考资料

图解设计模式

大话设计模式

设计模式之禅

github我见过最好的设计模式

http://c.biancheng.net/view/1326.html

定义

• 代理模式为其他对象提供一种代理，用来控制对于这个对象的访问
• 在客户类和目标类之间起到中介作用
• 结构型设计模式

类图

image-20210103193938980

生活中的代理模式

• 房产中介
• 快递小哥
• 黄牛党

使用场景

• 保护目标对象
• 增强目标对象的功能

案例

静态代理

显示声明被代理的对象

image-20210103191744391

public class ZhangLaosan implements IPerson {

  private IPerson zhangsan;

  public ZhangLaosan(IPerson person) {
    this.zhangsan = person;
  }

  public void findLove() {
    System.out.println("张老三开始物色");
    person.findLove();
    System.out.println("开始交往");
  }

}

编译期就清楚了

有点类似于装饰者模式

动态代理

JDK

• Proxy#newProxyInstance
• InvocationHandler

image-20210103192839331

public class JdkMeipo implements InvocationHandler {
  private IPerson target;
  public IPerson getInstance(IPerson target){
    this.target = target;
    Class<?> clazz =  target.getClass();
    return (IPerson) Proxy.newProxyInstance(clazz.getClassLoader(),clazz.getInterfaces(),this);
  }

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    before();
    Object result = method.invoke(this.target,args);
    after();
    return result;
  }

  private void after() {
    System.out.println("双方同意，开始交往");
  }

  private void before() {
    System.out.println("我是媒婆，已经收集到你的需求，开始物色");
  }
}

问题

好像动态代理和静态代理没有区别

• 动态代理编译期确认的，没有硬编码

cglibProxy

导入依赖

<dependency>
  <groupId>cglib</groupId>
  <artifactId>cglib-nodep</artifactId>
  <version>2.2</version>
</dependency>

• 实现net.sf.cglib.proxy.MethodInterceptor

public class CGlibMeipo implements MethodInterceptor {

  public Object getInstance(Class<?> clazz) throws Exception{
    //相当于Proxy，代理的工具类
    Enhancer enhancer = new Enhancer();
    enhancer.setSuperclass(clazz);
    enhancer.setCallback(this);
    return enhancer.create();
  }

  public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
    before();
    Object obj = methodProxy.invokeSuper(o,objects);
    after();
    return obj;
  }

  private void before(){
    System.out.println("我是媒婆，我要给你找对象，现在已经确认你的需求");
    System.out.println("开始物色");
  }

  private void after(){
    System.out.println("OK的话，准备办事");
  }
}

一般$开头的类都是动态代理的类

深入了解代理模式

JDK的动态代理

通过反编译来查看字节码文件

    public static void generator() {
        byte[] bytes = ProxyGenerator.generateProxyClass("$proxy0", new Class[]{IPerson.class});
        FileOutputStream os = null;
        try {
            os = new FileOutputStream("/Users/zzy/Downloads/$proxy0.class");
            os.write(bytes);
            os.close();
            System.out.println("print success");

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (os != null) {
                try {
                    os.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

反编译对应文件

jad $Proxy0.class

jad安装步骤可以参考https://blog.csdn.net/qing_gee/article/details/86510704

也可以直接放到idea中查看

编译后的内容如下

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

import com.zzjson.pattern.proxy.dynamicproxy.jdkproxy.IPerson;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $proxy0 extends Proxy implements IPerson {
  private static Method m1;
  private static Method m4;
  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 void findLove() throws  {
    try {
      super.h.invoke(this, m4, (Object[])null);
    } catch (RuntimeException | Error var2) {
      throw var2;
    } catch (Throwable var3) {
      throw new UndeclaredThrowableException(var3);
    }
  }

  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 void buyInsure() throws  {
    try {
      super.h.invoke(this, m3, (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"));
      m4 = Class.forName(" com.zzjson.pattern.proxy.dynamicproxy.jdkproxy.IPerson").getMethod("findLove");
      m2 = Class.forName("java.lang.Object").getMethod("toString");
      m3 = Class.forName(" com.zzjson.pattern.proxy.dynamicproxy.jdkproxy.IPerson").getMethod("buyInsure");
      m0 = Class.forName("java.lang.Object").getMethod("hashCode");
    } catch (NoSuchMethodException var2) {
      throw new NoSuchMethodError(var2.getMessage());
    } catch (ClassNotFoundException var3) {
      throw new NoClassDefFoundError(var3.getMessage());
    }
  }
}

关键代码

public final void findLove() throws  {
  try {
    super.h.invoke(this, m4, (Object[])null);
  } catch (RuntimeException | Error var2) {
    throw var2;
  } catch (Throwable var3) {
    throw new UndeclaredThrowableException(var3);
  }
}

h实际上就是代理对象

所以说实际上就是调用的

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  before();
  Object result = method.invoke(this.target,args);
  after();
  return result;
}

手写

1. 动态生成源码.java文件

   1. 自己手动拼代码

2. Java文件输出到磁盘，保存为文件$Proxy0.java

   String filePath = MyProxy.class.getResource("").getPath();
   //           System.out.println(filePath);
   File f = new File(filePath + "$Proxy0.java");
   FileWriter fw = new FileWriter(f);
   fw.write(src);
   fw.flush();
   fw.close();
   

3. 把.java文件编译成$proxy0.class文件

   JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
   StandardJavaFileManager manage = compiler.getStandardFileManager(null,null,null);
   Iterable iterable = manage.getJavaFileObjects(f);
   
   JavaCompiler.CompilationTask task = compiler.getTask(null,manage,null,null,null,iterable);
   task.call();
   manage.close();
   

4. 把生成的.class文件加载到JVM中

   Class proxyClass =  classLoader.findClass("$Proxy0");
   Constructor c = proxyClass.getConstructor(MyInvocationHandler.class);
   f.delete();
   

5. 返回新的代理对象

    return c.newInstance(h);
   

自己的类加载器

public class MyClassLoader extends ClassLoader {

  private File classPathFile;
  public MyClassLoader(){
    String classPath = MyClassLoader.class.getResource("").getPath();
    this.classPathFile = new File(classPath);
  }

  @Override
  protected Class<?> findClass(String name) throws ClassNotFoundException {

    String className = MyClassLoader.class.getPackage().getName() + "." + name;
    if(classPathFile  != null){
      File classFile = new File(classPathFile,name.replaceAll("\\.","/") + ".class");
      if(classFile.exists()){
        FileInputStream in = null;
        ByteArrayOutputStream out = null;
        try{
          in = new FileInputStream(classFile);
          out = new ByteArrayOutputStream();
          byte [] buff = new byte[1024];
          int len;
          while ((len = in.read(buff)) != -1){
            out.write(buff,0,len);
          }
          return defineClass(className,out.toByteArray(),0,out.size());
        }catch (Exception e){
          e.printStackTrace();
        }
      }
    }
    return null;
  }
}

CGLIb和DJK的区别

• CGLIB采用继承的方式，重写父类的方法
• JDK采用的方式，要求代理的目标对象一定要实现一个接口

思想：都是通过生成字节码，重组成一个新的类

• JDK Proxy对于用户而言，依赖性更强，调用也更加安全
• CGLIB要求目标类不能有final的方法，final方法不能被代理
• CGLIB更快
  • 效率更高，性能也更高，底层没有用到反射
• JDK的生成逻辑较为简单，执行效率姚笛，每次都要用反射

常见的动态代理

Spring

AOP

BeanPostProcessor核心还是在对象后置处理器

AbstractAdvisingBeanPostProcessor#postProcessAfterInitialization`

org.springframework.aop.framework.AbstractAdvisingBeanPostProcessor#postProcessAfterInitialization

public Object postProcessAfterInitialization(Object bean, String beanName) {
  if (this.advisor == null || bean instanceof AopInfrastructureBean) {
    // Ignore AOP infrastructure such as scoped proxies.
    return bean;
  }

  if (bean instanceof Advised) {
    Advised advised = (Advised) bean;
    if (!advised.isFrozen() && isEligible(AopUtils.getTargetClass(bean))) {
      // Add our local Advisor to the existing proxy's Advisor chain...
      if (this.beforeExistingAdvisors) {
        advised.addAdvisor(0, this.advisor);
      }
      else {
        advised.addAdvisor(this.advisor);
      }
      return bean;
    }
  }

  if (isEligible(bean, beanName)) {
    ProxyFactory proxyFactory = prepareProxyFactory(bean, beanName);
    if (!proxyFactory.isProxyTargetClass()) {
      evaluateProxyInterfaces(bean.getClass(), proxyFactory);
    }
    proxyFactory.addAdvisor(this.advisor);
    customizeProxyFactory(proxyFactory);
    return proxyFactory.getProxy(getProxyClassLoader());
  }

  // No proxy needed.
  return bean;
}

最终会执行到

DefaultAopProxyFactory#createAopProxy

org.springframework.aop.framework.DefaultAopProxyFactory#createAopProxy

    public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
        if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
            Class<?> targetClass = config.getTargetClass();
            if (targetClass == null) {
                throw new AopConfigException("TargetSource cannot determine target class: " +
                        "Either an interface or a target is required for proxy creation.");
            }
            if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
                return new JdkDynamicAopProxy(config);
            }
            return new ObjenesisCglibAopProxy(config);
        }
        else {
            return new JdkDynamicAopProxy(config);
        }
    }

JDK动态代理

public Object getProxy(@Nullable ClassLoader classLoader) {
  if (logger.isTraceEnabled()) {
    logger.trace("Creating JDK dynamic proxy: " + this.advised.getTargetSource());
  }
  Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised, true);
  findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
  return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}

对应的InvocationHandler

org.springframework.aop.framework.JdkDynamicAopProxy#invoke

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  Object oldProxy = null;
  boolean setProxyContext = false;

  TargetSource targetSource = this.advised.targetSource;
  Object target = null;

  try {
    if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
      // The target does not implement the equals(Object) method itself.
      return equals(args[0]);
    }
    else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
      // The target does not implement the hashCode() method itself.
      return hashCode();
    }
    else if (method.getDeclaringClass() == DecoratingProxy.class) {
      // There is only getDecoratedClass() declared -> dispatch to proxy config.
      return AopProxyUtils.ultimateTargetClass(this.advised);
    }
    else if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
             method.getDeclaringClass().isAssignableFrom(Advised.class)) {
      // Service invocations on ProxyConfig with the proxy config...
      return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
    }

    Object retVal;

    if (this.advised.exposeProxy) {
      // Make invocation available if necessary.
      oldProxy = AopContext.setCurrentProxy(proxy);
      setProxyContext = true;
    }

    // Get as late as possible to minimize the time we "own" the target,
    // in case it comes from a pool.
    target = targetSource.getTarget();
    Class<?> targetClass = (target != null ? target.getClass() : null);

    // Get the interception chain for this method.
    List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

    // Check whether we have any advice. If we don't, we can fallback on direct
    // reflective invocation of the target, and avoid creating a MethodInvocation.
    if (chain.isEmpty()) {
      // We can skip creating a MethodInvocation: just invoke the target directly
      // Note that the final invoker must be an InvokerInterceptor so we know it does
      // nothing but a reflective operation on the target, and no hot swapping or fancy proxying.
      Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
      retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
    }
    else {
      // We need to create a method invocation...
      MethodInvocation invocation =
        new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
      // Proceed to the joinpoint through the interceptor chain.
      retVal = invocation.proceed();
    }

    // Massage return value if necessary.
    Class<?> returnType = method.getReturnType();
    if (retVal != null && retVal == target &&
        returnType != Object.class && returnType.isInstance(proxy) &&
        !RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
      // Special case: it returned "this" and the return type of the method
      // is type-compatible. Note that we can't help if the target sets
      // a reference to itself in another returned object.
      retVal = proxy;
    }
    else if (retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) {
      throw new AopInvocationException(
        "Null return value from advice does not match primitive return type for: " + method);
    }
    return retVal;
  }
  finally {
    if (target != null && !targetSource.isStatic()) {
      // Must have come from TargetSource.
      targetSource.releaseTarget(target);
    }
    if (setProxyContext) {
      // Restore old proxy.
      AopContext.setCurrentProxy(oldProxy);
    }
  }
}

CGLIB动态代理

public Object getProxy(@Nullable ClassLoader classLoader) {
  if (logger.isTraceEnabled()) {
    logger.trace("Creating CGLIB proxy: " + this.advised.getTargetSource());
  }

  try {
    Class<?> rootClass = this.advised.getTargetClass();
    Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");

    Class<?> proxySuperClass = rootClass;
    if (rootClass.getName().contains(ClassUtils.CGLIB_CLASS_SEPARATOR)) {
      proxySuperClass = rootClass.getSuperclass();
      Class<?>[] additionalInterfaces = rootClass.getInterfaces();
      for (Class<?> additionalInterface : additionalInterfaces) {
        this.advised.addInterface(additionalInterface);
      }
    }

    // Validate the class, writing log messages as necessary.
    validateClassIfNecessary(proxySuperClass, classLoader);

    // Configure CGLIB Enhancer...
    Enhancer enhancer = createEnhancer();
    if (classLoader != null) {
      enhancer.setClassLoader(classLoader);
      if (classLoader instanceof SmartClassLoader &&
          ((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
        enhancer.setUseCache(false);
      }
    }
    enhancer.setSuperclass(proxySuperClass);
    enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
    enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
    enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(classLoader));

    Callback[] callbacks = getCallbacks(rootClass);
    Class<?>[] types = new Class<?>[callbacks.length];
    for (int x = 0; x < types.length; x++) {
      types[x] = callbacks[x].getClass();
    }
    // fixedInterceptorMap only populated at this point, after getCallbacks call above
    enhancer.setCallbackFilter(new ProxyCallbackFilter(
      this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
    enhancer.setCallbackTypes(types);

    // Generate the proxy class and create a proxy instance.
    return createProxyClassAndInstance(enhancer, callbacks);
  }
  catch (CodeGenerationException | IllegalArgumentException ex) {
    throw new AopConfigException("Could not generate CGLIB subclass of " + this.advised.getTargetClass() +
                                 ": Common causes of this problem include using a final class or a non-visible class",
                                 ex);
  }
  catch (Throwable ex) {
    // TargetSource.getTarget() failed
    throw new AopConfigException("Unexpected AOP exception", ex);
  }
}

对应的Interceptor

org.springframework.aop.framework.CglibAopProxy.DynamicAdvisedInterceptor#intercept

public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
  Object oldProxy = null;
  boolean setProxyContext = false;
  Object target = null;
  TargetSource targetSource = this.advised.getTargetSource();
  try {
    if (this.advised.exposeProxy) {
      // Make invocation available if necessary.
      oldProxy = AopContext.setCurrentProxy(proxy);
      setProxyContext = true;
    }
    // Get as late as possible to minimize the time we "own" the target, in case it comes from a pool...
    target = targetSource.getTarget();
    Class<?> targetClass = (target != null ? target.getClass() : null);
    List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
    Object retVal;
    // Check whether we only have one InvokerInterceptor: that is,
    // no real advice, but just reflective invocation of the target.
    if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {
      // We can skip creating a MethodInvocation: just invoke the target directly.
      // Note that the final invoker must be an InvokerInterceptor, so we know
      // it does nothing but a reflective operation on the target, and no hot
      // swapping or fancy proxying.
      Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
      retVal = methodProxy.invoke(target, argsToUse);
    }
    else {
      // We need to create a method invocation...
      retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
    }
    retVal = processReturnType(proxy, target, method, retVal);
    return retVal;
  }
  finally {
    if (target != null && !targetSource.isStatic()) {
      targetSource.releaseTarget(target);
    }
    if (setProxyContext) {
      // Restore old proxy.
      AopContext.setCurrentProxy(oldProxy);
    }
  }
}

事物

org.springframework.transaction.interceptor.TransactionInterceptor#invoke

org.springframework.transaction.interceptor.TransactionAspectSupport#invokeWithinTransaction

我们在Spring中可以使用声明式事物和编程式事物

• @Transactional
• 和编程式事物

多数据源的切换

org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource

缓存

org.springframework.cache.interceptor.CacheInterceptor#invoke

• Cacheable
• ``CacheConfig`
• CacheEvict
• CachePut
• Caching
• EnableCaching

Mybatis

为什么通过Mapper接口就能够调用，不需要写实现类

Mapper接口

创建MapperProxyFactory

org.apache.ibatis.binding.MapperRegistry#addMapper

  public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
      if (hasMapper(type)) {
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      }
      boolean loadCompleted = false;
      try {
        knownMappers.put(type, new MapperProxyFactory<T>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

这一段先初始化MapperProxyFactory，并且解析mapper接口上的注解

获取Mapper

org.apache.ibatis.binding.MapperRegistry#getMapper

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 {
    return mapperProxyFactory.newInstance(sqlSession);
  } catch (Exception e) {
    throw new BindingException("Error getting mapper instance. Cause: " + e, e);
  }
}

当前获取代理对象通过MapperProxyFactory来生成的

代理

org.apache.ibatis.binding.MapperProxyFactory#newInstance(org.apache.ibatis.binding.MapperProxy<T>)

@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
  return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}

public T newInstance(SqlSession sqlSession) {
  final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
  return newInstance(mapperProxy);
}

查看对应的代理，可以看到通过Proxy#newProxyInstance来生成的代理对象，也就是走的JDK的动态代理

具体的Invocationhandler

public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;

  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }

}

具体的执行是执行的MapperProxy#invoke方法

所以Mapper接口有要求，不能是final类并且方法不能是final的，并且方法不能重载

Spring Cloud Feign

如果了解过Spring Cloud Feign应该可以知道可以在对应的接口上面加FeignClient那么在调用的时候就会帮我们自动负载均衡

@FeignClient(
    value = AppConstant.APPLICATION_USER_NAME
)
public interface IUserClient {
    /**
     * 获取用户信息
     *
     * @param userId 用户id
     * @return
     */
    @GetMapping(USER_INFO_BY_ID)
    R<User> userInfoById(@RequestParam("userId") Long userId);

肯定是做了代理，那么我们查看代码探究一下Spring Cloud Feign是如何做的

创建FeignClientFactoryBean

org.springframework.cloud.openfeign.FeignClientsRegistrar#registerFeignClient

可以看到

BeanDefinitionBuilder definition = BeanDefinitionBuilder
  .genericBeanDefinition(FeignClientFactoryBean.class);

创建了一个FeignClientFactoryBean所以会调用他的

初始化生成代理对象

org.springframework.cloud.openfeign.FeignClientFactoryBean#getObject

    public Object getObject() throws Exception {
        return getTarget();
    }

可以看到getTarget

<T> T getTarget() {
  FeignContext context = applicationContext.getBean(FeignContext.class);
  Feign.Builder builder = feign(context);
//省略
  String url = this.url + cleanPath();
  Client client = getOptional(context, Client.class);
  if (client != null) {
    if (client instanceof LoadBalancerFeignClient) {
      // not load balancing because we have a url,
      // but ribbon is on the classpath, so unwrap
      client = ((LoadBalancerFeignClient) client).getDelegate();
    }
    if (client instanceof FeignBlockingLoadBalancerClient) {
      // not load balancing because we have a url,
      // but Spring Cloud LoadBalancer is on the classpath, so unwrap
      client = ((FeignBlockingLoadBalancerClient) client).getDelegate();
    }
    builder.client(client);
  }
  Targeter targeter = get(context, Targeter.class);
  return (T) targeter.target(this, builder, context,
                             new HardCodedTarget<>(type, name, url));
}

调用了targeter#target来生成代理对象

public <T> T target(Target<T> target) {
return build().newInstance(target);
}

生成代理

feign.ReflectiveFeign#newInstance

public <T> T newInstance(Target<T> target) {
  Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
  Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
  List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();

  for (Method method : target.type().getMethods()) {
    if (method.getDeclaringClass() == Object.class) {
      continue;
    } else if (Util.isDefault(method)) {
      DefaultMethodHandler handler = new DefaultMethodHandler(method);
      defaultMethodHandlers.add(handler);
      methodToHandler.put(method, handler);
    } else {
      methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
    }
  }
  InvocationHandler handler = factory.create(target, methodToHandler);
  T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),
                                       new Class<?>[] {target.type()}, handler);

  for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
    defaultMethodHandler.bindTo(proxy);
  }
  return proxy;
}

可以看到也是通过JDK的动态代理来生成代理对象的

InvocationHandler如下

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  if ("equals".equals(method.getName())) {
    try {
      Object otherHandler =
        args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;
      return equals(otherHandler);
    } catch (IllegalArgumentException e) {
      return false;
    }
  } else if ("hashCode".equals(method.getName())) {
    return hashCode();
  } else if ("toString".equals(method.getName())) {
    return toString();
  }

  return dispatch.get(method).invoke(args);
}

会通过方法来分发做负载均衡

总结

优点

• 能把代理对象和真实被调用的目标对象分离
• 降低了系统的耦合度，易于扩展
• 能够保护目标对象
• 增强目标对象的功能

缺点

• 系统类数目增加
• 客户端和目标对象之间增加了代理对象，请求处理速度会变慢，增加了系统的复杂度

Spring中的代理选择原则

1. Bean中有实现接口的时候，Spring会使用JDK的动态代理
2. Bean没有实现接口的时候，Spring会选择CGLIB
3. Spring可以通过配置强制使用CGLIB,只需在Spring的配置文件中加入这个代码

<aop:aspectj-autoproxy proxy-target-class="true">

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