配景

在RPC接口挪用场景可能利用动态署理的场景中，偶然会呈现UndeclaredThrowableException，又可能在利用反射的场景中，呈现InvocationTargetException，这都与我们所期望的异常纷歧致，且将真实的异常信息埋没在更深一层的仓库中。本文将重点阐明下UndeclaredThrowableException

先给结论

利用jdk动态署理接口时，若要领执行进程中抛出了受检异常但要领签名又没有声明该异常时则会被署理类包装成UndeclaredThrowableException抛出。

问题还原

// 接口界说
public interface IService {
    void foo() throws SQLException;
}
public class ServiceImpl implements IService{
    @Override
    public void foo() throws SQLException {
        throw new SQLException("I test throw an checked Exception");
    }
}
// 动态署理
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 {
        return method.invoke(target, args);
    }
}

运行上面的MainTest，获得的异常仓库为

java.lang.reflect.UndeclaredThrowableException
	at com.sun.proxy.$Proxy0.foo(Unknown Source)
	at com.learn.reflect.MainTest.main(MainTest.java:16)
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.learn.reflect.IServiceProxy.invoke(IServiceProxy.java:19)
	... 2 more
Caused by: java.sql.SQLException: I test throw an checked Exception
	at com.learn.reflect.ServiceImpl.foo(ServiceImpl.java:11)
	... 7 more

而我们期望的是

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

原因阐明

在上述问题还原中，真实的SQLException被包装了两层，先被InvocationTargetException包装，再被UndeclaredThrowableException包装。 个中，InvocationTargetException为受检异常，UndeclaredThrowableException为运行时异常。 为何会被包装呢，还要从动态署理的生成的署理类说起。

jdk动态署剖析在运行时生成委托接口的详细实现类，我们通过ProxyGenerator手动生成下class文件，再操作idea理会class文件获得详细署理类： 截取部门：

public final class IServiceProxy$1 extends Proxy implements IService {
    private static Method m1;
    private static Method m2;
    private static Method m3;
    private static Method m0;

    public IServiceProxy$1(InvocationHandler var1) throws  {
        super(var1);
    }
	
	public final void foo() throws SQLException {
        try {
            super.h.invoke(this, m3, (Object[])null);
        } catch (RuntimeException | SQLException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }
	static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[]{Class.forName("java.lang.Object")});
            m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
            m3 = Class.forName("com.learn.reflect.IService").getMethod("foo", new Class[0]);
            m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}