Java中使用JDK代理时避免UndeclaredThrowab

当InvocationHandler抛出运行时异常时，异常会被直接抛出。
但如果抛出未在接口中声明的受检异常，就会被包装成UndeclaredThrowableException。

最常见的是代理真实对象时，直接抛出被代理对象的InvocationTargetException异常。
对于接口中声明的异常，代理也会体现在生成的代理类里。

final class $Proxy4 extends Proxy implements MyInterface {
   private static Method m3;
   public $Proxy4(InvocationHandler var1) throws  {super(var1);}
   public final String test() throws IOException {
      try {
         return (String)super.h.invoke(this, m3, (Object[])null);
      } catch (RuntimeException | IOException | Error var2) {
         throw var2;
      } catch (Throwable var3) {
         throw new UndeclaredThrowableException(var3);
      }
   }
   static {
      try {
         m3 = Class.forName("MyTest.proxy.MyInterface").getMethod("test");
      } catch (NoSuchMethodException var2) {
         throw new NoSuchMethodError(var2.getMessage());
      } catch (ClassNotFoundException var3) {
         throw new NoClassDefFoundError(var3.getMessage());
      }
   }
}

我们可以在ProxyGenerator#addProxyMethod看到处理声明异常的逻辑

private void addProxyMethod(Method var1, Class<?> var2) {
        String var3 = var1.getName();
        Class[] var4 = var1.getParameterTypes();
        Class var5 = var1.getReturnType();
        Class[] var6 = var1.getExceptionTypes();
        String var7 = var3 + getParameterDescriptors(var4);
        Object var8 = (List)this.proxyMethods.get(var7);
        if (var8 != null) {
            Iterator var9 = ((List)var8).iterator();
            while(var9.hasNext()) {
                ProxyGenerator.ProxyMethod var10 = (ProxyGenerator.ProxyMethod)var9.next();
                if (var5 == var10.returnType) {
                    ArrayList var11 = new ArrayList();
                    collectCompatibleTypes(var6, var10.exceptionTypes, var11);
                    collectCompatibleTypes(var10.exceptionTypes, var6, var11);
                    var10.exceptionTypes = new Class[var11.size()];
                    var10.exceptionTypes = (Class[])var11.toArray(var10.exceptionTypes);
                    return;
                }
            }
        } else {
            var8 = new ArrayList(3);
            this.proxyMethods.put(var7, var8);
        }
        ((List)var8).add(new ProxyGenerator.ProxyMethod(var3, var4, var5, var6, var2));
    }

对于动态生成的代理类，可以在ProxyGenerator#generateProxyClass看到，如果设置环境变量sun.misc.ProxyGenerator.saveGeneratedFiles=true，那么该类会将生成的代理类写入到文件中，如下：

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) {
            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;
    }

参见 https://stackoverflow.com/questions/3715298/dynamic-proxy-and-checked-exceptions