Java进阶—动态代理场景下抛出UndeclaredThrowableException

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| 字数统计: 2,805

问题

在SpringBoot项目中调用dubbo服务时，使用了Spring的aop对dubbo调用增加了切面功能。在切面类中本来想对异常进行统一处理，抛出项目的统一异常ServiceException，但切面方法抛出的确是UndeclaredThrowableException异常。

几个关键的Java类如下：
切面类：

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@Aspect
@Component
@Slf4j
public class DubboAspect {


    @Around(value = "execution(public * com.lzumetal.springboot.aop.dubbo.OrderInvoker.*(..))")
    public Object aroundAdvice(ProceedingJoinPoint joinPoint) throws Throwable {
        long start = System.currentTimeMillis();
        String name = joinPoint.getSignature().getName();
        Object proceed;
        try {
            proceed = joinPoint.proceed();
        } catch (Exception e) {
            String errMsg = "服务器错误";
            if (e instanceof OrderServiceException) {
                errMsg = e.getMessage();
            }
            throw new ServiceException(500, errMsg);
        } finally {
            long end = System.currentTimeMillis();
            long time = end - start;
            log.info("dubbo服务调用耗时|method={}|{}ms", name, time);
        }
        return proceed;
    }


}

dubbo服务调用类（模拟）：

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@Service
@Slf4j
public class OrderInvoker {



    public Order queryById(Long id) {
        log.info("模拟调用dubbo服务...");
        throw new OrderServiceException("订单不存在");
    }

    
}

项目统一异常：

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public class ServiceException extends Exception {

    private int code;

    private String msg;

    public ServiceException(int code, String msg) {
        super(msg);
        this.code = code;
    }

}

dubbo服务抛出的异常：

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public class OrderServiceException extends RuntimeException {

    public OrderServiceException(String message) {
        super(message);
    }

}

单元测试方法：

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@Test
public void testQueryOrder() {
    Order order = orderInvoker.queryById(1L);
    log.info("queryOrder|{}", order);
}

输出结果如下：

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2020-07-08 09:28:30.740  INFO 39232 --- [           main] c.l.springboot.aop.dubbo.OrderInvoker    : 模拟调用dubbo服务...
2020-07-08 09:28:30.740  INFO 39232 --- [           main] c.l.springboot.aop.aspect.DubboAspect    : dubbo服务调用耗时|method=queryById|10ms

java.lang.reflect.UndeclaredThrowableException
	at com.lzumetal.springboot.aop.dubbo.OrderInvoker$$EnhancerBySpringCGLIB$$ce9b2886.queryById(<generated>)
	at com.lzumetal.springboot.aop.test.MainTest.testQueryOrder(MainTest.java:29)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.springframework.test.context.junit4.statements.RunBeforeTestExecutionCallbacks.evaluate(RunBeforeTestExecutionCallbacks.java:74)
	at org.springframework.test.context.junit4.statements.RunAfterTestExecutionCallbacks.evaluate(RunAfterTestExecutionCallbacks.java:84)
	at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
	at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
	at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:251)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
	at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: com.lzumetal.springboot.aop.exception.ServiceException: 订单不存在
	at com.lzumetal.springboot.aop.aspect.DubboAspect.aroundAdvice(DubboAspect.java:33)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethodWithGivenArgs(AbstractAspectJAdvice.java:644)
	at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethod(AbstractAspectJAdvice.java:633)
	at org.springframework.aop.aspectj.AspectJAroundAdvice.invoke(AspectJAroundAdvice.java:70)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
	at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:93)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:688)
	... 32 more

原因

因为Spring aop中的切面类用到了jdk中的动态代理，而在动态代理的实现中，如果调用过程中抛出的异常是检查型异常，并且没有在被代理的接口中声明，则会将异常包装成UndeclaredThrowableException并抛出。

查看InvocationHandler接口的源码，可以看到invoke方法的javadoc注释对抛出的Throwable有明确说明：

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public interface InvocationHandler {

    /**
     * Processes a method invocation on a proxy instance and returns
     * the result.  This method will be invoked on an invocation handler
     * when a method is invoked on a proxy instance that it is
     * associated with.
     *
     * @param   proxy the proxy instance that the method was invoked on
     *
     * @param   method the {@code Method} instance corresponding to
     * the interface method invoked on the proxy instance.  The declaring
     * class of the {@code Method} object will be the interface that
     * the method was declared in, which may be a superinterface of the
     * proxy interface that the proxy class inherits the method through.
     *
     * @param   args an array of objects containing the values of the
     * arguments passed in the method invocation on the proxy instance,
     * or {@code null} if interface method takes no arguments.
     * Arguments of primitive types are wrapped in instances of the
     * appropriate primitive wrapper class, such as
     * {@code java.lang.Integer} or {@code java.lang.Boolean}.
     *
     * @return  the value to return from the method invocation on the
     * proxy instance.  If the declared return type of the interface
     * method is a primitive type, then the value returned by
     * this method must be an instance of the corresponding primitive
     * wrapper class; otherwise, it must be a type assignable to the
     * declared return type.  If the value returned by this method is
     * {@code null} and the interface method's return type is
     * primitive, then a {@code NullPointerException} will be
     * thrown by the method invocation on the proxy instance.  If the
     * value returned by this method is otherwise not compatible with
     * the interface method's declared return type as described above,
     * a {@code ClassCastException} will be thrown by the method
     * invocation on the proxy instance.
     *
     * @throws  Throwable the exception to throw from the method
     * invocation on the proxy instance.  The exception's type must be
     * assignable either to any of the exception types declared in the
     * {@code throws} clause of the interface method or to the
     * unchecked exception types {@code java.lang.RuntimeException}
     * or {@code java.lang.Error}.  If a checked exception is
     * thrown by this method that is not assignable to any of the
     * exception types declared in the {@code throws} clause of
     * the interface method, then an
     * {@link UndeclaredThrowableException} containing the
     * exception that was thrown by this method will be thrown by the
     * method invocation on the proxy instance.
     *
     * @see     UndeclaredThrowableException
     */
    public Object invoke(Object proxy, Method method, Object[] args)
        throws Throwable;
}

现在，我们来通过简单代码演示一下动态代理抛出UndeclaredThrowableException的过程，还原前面在项目中遇到的那个问题。

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// 接口定义
public interface IService {
    void foo() throws SQLException;
}
// 接口实现
public class ServiceImpl implements IService{
    @Override
    public void foo() throws SQLException {
        throw new SQLException("I test throw an checked Exception");
    }
}
// 动态代理
public class IServiceProxy implements InvocationHandler {
    private Object target;

    IServiceProxy(Object target){
        this.target = target;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    	System.out.println("入参：" + Arrays.toString(args));
        return method.invoke(target, args);
    }
}

public class MainTest {
    public static void main(String[] args) {
        IService service = new ServiceImpl();
        IService serviceProxy = (IService) Proxy.newProxyInstance(service.getClass().getClassLoader(),
                service.getClass().getInterfaces(), 
                new IServiceProxy(service));
        try {
            serviceProxy.foo();
        } catch (Exception e){
            e.printStackTrace();
        }
    }
}

运行上面的MainTest，得到的异常堆栈为

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入参：null
java.lang.reflect.UndeclaredThrowableException
	at com.sun.proxy.$Proxy0.foo(Unknown Source)
	at com.lzumetal.java.learn.proxy.MainTest.main(MainTest.java:20)
Caused by: java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.lzumetal.java.learn.proxy.IServiceProxy.invoke(IServiceProxy.java:23)
	... 2 more
Caused by: java.sql.SQLException: throw an unchecked exception
	at com.lzumetal.java.learn.proxy.service.ServiceImpl.foo(ServiceImpl.java:13)
	... 7 more

而我们本来期望的是抛出：

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java.sql.SQLException: I test throw an checked Exception
    at com.learn.reflect.ServiceImpl.foo(ServiceImpl.java:11)
    ...

为什么异常会这样抛出呢？为了再进一步找到更深层次的原因，我们使用JDK自带的ProxyGenerator类的generateProxyClass()方法来生成代理类的字节码（.class文件）。

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package com.lzumetal.java.learn.proxy;

import com.lzumetal.java.learn.proxy.service.ServiceImpl;
import sun.misc.ProxyGenerator;

import java.io.FileOutputStream;

public class ProxyGeneratorUtil {

    private static String DEFAULT_CLASS_NAME = "$Proxy";

    public static byte[] saveGenerateProxyClass(String path, Class<?>[] interfaces) {

        byte[] classFile = ProxyGenerator.generateProxyClass(DEFAULT_CLASS_NAME, interfaces);
        String filePath = path + "/" + DEFAULT_CLASS_NAME + ".class";
        try (FileOutputStream out = new FileOutputStream(filePath)) {
            out.write(classFile);
            out.flush();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return classFile;
    }

    public static void main(String[] args) {

        Class<?> interfaces[] = {ServiceImpl.class};
        //运行时，确保目录存在
        saveGenerateProxyClass("d:\\", interfaces);


    }

}

通过Luyten工具，查看字节码反编译后的代码，截取一部分如下：

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 public final void foo() throws SQLException {
     try {
         super.h.invoke(this, $Proxy.m3, null);
     }
     catch (Error | RuntimeException | SQLException error) {
         throw;
     }
     catch (Throwable t) {
         throw new UndeclaredThrowableException(t);
     }
 }

static {
     try {
         $Proxy.m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
         $Proxy.m8 = Class.forName("com.lzumetal.java.learn.proxy.service.ServiceImpl").getMethod("notify", (Class<?>[])new Class[0]);
         $Proxy.m2 = Class.forName("java.lang.Object").getMethod("toString", (Class<?>[])new Class[0]);
         $Proxy.m3 = Class.forName("com.lzumetal.java.learn.proxy.service.ServiceImpl").getMethod("foo", (Class<?>[])new Class[0]);
         $Proxy.m6 = Class.forName("com.lzumetal.java.learn.proxy.service.ServiceImpl").getMethod("wait", Long.TYPE);
         $Proxy.m5 = Class.forName("com.lzumetal.java.learn.proxy.service.ServiceImpl").getMethod("wait", Long.TYPE, Integer.TYPE);
         $Proxy.m7 = Class.forName("com.lzumetal.java.learn.proxy.service.ServiceImpl").getMethod("getClass", (Class<?>[])new Class[0]);
         $Proxy.m9 = Class.forName("com.lzumetal.java.learn.proxy.service.ServiceImpl").getMethod("notifyAll", (Class<?>[])new Class[0]);
         $Proxy.m0 = Class.forName("java.lang.Object").getMethod("hashCode", (Class<?>[])new Class[0]);
         $Proxy.m4 = Class.forName("com.lzumetal.java.learn.proxy.service.ServiceImpl").getMethod("wait", (Class<?>[])new Class[0]);
     }
     catch (NoSuchMethodException ex) {
         throw new NoSuchMethodError(ex.getMessage());
     }
     catch (ClassNotFoundException ex2) {
         throw new NoClassDefFoundError(ex2.getMessage());
     }
 }

从上面的字节码可以看出，代理类抛出异常的逻辑和invoke(Object proxy, Method method, Object[] args)方法上的注释是一致的：对RuntimeException、接口已声明的异常、Error直接抛出，其他异常被包装成UndeclaredThrowableException抛出。
但另一个疑问也来了，SQLException是一个检查型异常，但我们在接口的方法中命名已经申明了抛出SQLException，为什么最终还是抛出了UndeclaredThrowableException？其原因其实在打印的堆栈中也能看出端倪了，即代理类在重写父接口的invoke(Object proxy, Method method, Object[] args)方法时，会去调用反射的method.invoke(target, args)方法，而这个方法是可能会抛出InvocationTargetException异常的：

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@CallerSensitive
public Object invoke(Object obj, Object... args)
    throws IllegalAccessException, IllegalArgumentException,
       InvocationTargetException
{
    if (!override) {
        if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {
            Class<?> caller = Reflection.getCallerClass();
            checkAccess(caller, clazz, obj, modifiers);
        }
    }
    MethodAccessor ma = methodAccessor;             // read volatile
    if (ma == null) {
        ma = acquireMethodAccessor();
    }
    return ma.invoke(obj, args);
}

当Method类的invoke方法抛出了InvocationTargetException异常，这个异常是受检异常，而代理类在处理异常时发现该异常在接口中没有声明，所以包装为UndeclaredThrowableException，并最终在代理类中抛出。

解决方法

知道了异常抛出的过程，就可以针对原因进行相应的解决了，比如上面的示例，可以在代理类的invoke(Object proxy, Method method, Object[] args)方法中对method.invoke(target, args)进行try-catch，抛出 InvocationTargetException的cause。

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@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    System.out.println("入参：" + Arrays.toString(args));
    try {
        return method.invoke(target, args);
    } catch (InvocationTargetException e) {
        throw e.getCause();
    }
}

而对于Spring AOP中代理类中抛出的UndeclaredThrowableException，因为这个代理类是Spring帮我们生成的，我们无法直接修改代理类，所以可以通过Spring的统一异常处理器进行处理。

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@ControllerAdvice
@Order(value = -9)
@Slf4j
public class DefaultExcpetionHandler {

    /**
     * aop中因动态代理抛出的自定义ServiceException异常会转换成UndeclaredThrowableException
     *
     * @param e
     * @return
     */
    @ExceptionHandler(UndeclaredThrowableException.class)
    @ResponseBody
    public Response handleUndeclaredThrowableException(UndeclaredThrowableException e) {
        Throwable throwable = e.getUndeclaredThrowable();
        log.error("处理UndeclaredThrowableException|", throwable);
        if (throwable instanceof ServiceException) {
            ServiceException exception = (ServiceException) e.getUndeclaredThrowable();
            return Response.data(exception.getErrorCode(), exception.getMessage(), null);
        }
        return Response.data(500, "服务器错误", null);
    }
}
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