java.lang.reflect.代理 : Huge exception stack trace

Question

这是一个简单的 Java 应用程序:

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class Main {

    interface MyInterface {
        void myMethod();
    }

    public static void main(String[] args) {
        MyInterface myInterface = (MyInterface) Proxy.newProxyInstance(Main.class.getClassLoader(), new Class[] {MyInterface.class},
            new InvocationHandler() {
                @Override
                public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                    return method.invoke(proxy, args);
                }
            });
        myInterface.myMethod();
    }
}

我希望在这里得到一个简单的 StackOverflowError，因为我在代理实例上递归调用相同的方法。

但是，异常产生的堆栈跟踪包含数百万行和数百 MB 的大小。

堆栈跟踪的第一部分是这样开始的:

java.lang.reflect.UndeclaredThrowableException
    at $Proxy0.myMethod(Unknown Source)
    at Main.main(Main.java:22)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at Main$1.invoke(Main.java:18)
    ... 2 more
Caused by: java.lang.reflect.UndeclaredThrowableException
    at $Proxy0.myMethod(Unknown Source)
    ... 7 more
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at Main$1.invoke(Main.java:18)
    ... 8 more

并扩展到:

Caused by: java.lang.reflect.UndeclaredThrowableException
    at $Proxy0.myMethod(Unknown Source)
    ... 1022 more

然后遵循数百万行，例如:

Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at Main$1.invoke(Main.java:18)
    at $Proxy0.myMethod(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at Main$1.invoke(Main.java:18)
    .......

和:

Caused by: java.lang.reflect.UndeclaredThrowableException
    at $Proxy0.myMethod(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at Main$1.invoke(Main.java:18)
    at $Proxy0.myMethod(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at Main$1.invoke(Main.java:18)
    .......

最后打印的行是(在上面的重复序列之后):

Exception: java.lang.StackOverflowError thrown from the UncaughtExceptionHandler in thread "main"

我怀疑堆栈跟踪生成机制正在调用代理实例上的一些方法(可能是 toString 来打印它，或者其他什么)，从而一遍又一遍地重复递归(因为每个方法调用在代理上导致无限递归)，但代理方法执行的总计数为 1919(通过在 InvocationHandler.invoke 方法中递增计数器来测量)。

在我的实际用例中，我当然解决了无限递归问题；我很好奇是否有人知道这是错误还是有合理的解释？

Java 版本:

java version "1.8.0_65"
Java(TM) SE Runtime Environment (build 1.8.0_65-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.65-b01, mixed mode)

编辑

@JiriTousek 和@AndrewWilliamson 友善地分析了这可能是什么原因。我根据他们的输入实现了模拟:

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.UndeclaredThrowableException;

public class Test {
    private static int counter = 0;

    public static void main(String[] args) {
        proxy();
    }

    private static void proxy() {
        try {
            methodInvoke();
        } catch (Throwable e) {
            throw new UndeclaredThrowableException(e);
        }
    }

    private static void methodInvoke() throws InvocationTargetException {
        try {
            myMethod();
        } catch (Throwable e) {
            throw new InvocationTargetException(e);
        }
    }

    private static void myMethod() {
        if (counter++ == 5) {
            throw new StackOverflowError();
        }
        proxy();
    }
}

这导致以下堆栈跟踪:

Exception in thread "main" java.lang.reflect.UndeclaredThrowableException
    at Test.proxy(Test.java:16)
    at Test.main(Test.java:9)
Caused by: java.lang.reflect.InvocationTargetException
    at Test.methodInvoke(Test.java:24)
    at Test.proxy(Test.java:14)
    ... 1 more
Caused by: java.lang.reflect.UndeclaredThrowableException
    at Test.proxy(Test.java:16)
    at Test.myMethod(Test.java:32)
    at Test.methodInvoke(Test.java:22)
    ... 2 more
Caused by: java.lang.reflect.InvocationTargetException
    at Test.methodInvoke(Test.java:24)
    at Test.proxy(Test.java:14)
    ... 4 more
Caused by: java.lang.reflect.UndeclaredThrowableException
    at Test.proxy(Test.java:16)
    at Test.myMethod(Test.java:32)
    at Test.methodInvoke(Test.java:22)
    ... 5 more
Caused by: java.lang.reflect.InvocationTargetException
    at Test.methodInvoke(Test.java:24)
    at Test.proxy(Test.java:14)
    ... 7 more
Caused by: java.lang.reflect.UndeclaredThrowableException
    at Test.proxy(Test.java:16)
    at Test.myMethod(Test.java:32)
    at Test.methodInvoke(Test.java:22)
    ... 8 more
Caused by: java.lang.reflect.InvocationTargetException
    at Test.methodInvoke(Test.java:24)
    at Test.proxy(Test.java:14)
    ... 10 more
Caused by: java.lang.reflect.UndeclaredThrowableException
    at Test.proxy(Test.java:16)
    at Test.myMethod(Test.java:32)
    at Test.methodInvoke(Test.java:22)
    ... 11 more
Caused by: java.lang.reflect.InvocationTargetException
    at Test.methodInvoke(Test.java:24)
    at Test.proxy(Test.java:14)
    ... 13 more
Caused by: java.lang.reflect.UndeclaredThrowableException
    at Test.proxy(Test.java:16)
    at Test.myMethod(Test.java:32)
    at Test.methodInvoke(Test.java:22)
    ... 14 more
Caused by: java.lang.reflect.InvocationTargetException
    at Test.methodInvoke(Test.java:24)
    at Test.proxy(Test.java:14)
    ... 16 more
Caused by: java.lang.StackOverflowError
    at Test.myMethod(Test.java:30)
    at Test.methodInvoke(Test.java:22)
    ... 17 more

因此，每个堆栈帧的行号都没有增长。

Answer 1

我对这个堆栈跟踪的解释是:

• 当堆栈溢出最终发生时，抛出了一个StackOverflowError
• 此错误被 method.invoke() 捕获并转换为 InvocationTargetException(根据 it's javadoc )，以原始异常为原因
• 由于这个异常是一个已检查的异常，Proxy 不能让它自己掉下来，所以它捕获了它并将它翻译成一个 UndeclaredThrowableException，再次将前一个作为原因

这样，对于堆栈溢出之前的每个递归级别，您都会得到另外两个“由”异常及其堆栈跟踪 - 大量输出。至于输出多少，大家猜猜看:

• 根据您的帖子大约有 2000 次调用
• 在每次调用中，堆栈跟踪似乎都增长了 5 行
• 为每次调用打印 2 个错误和堆栈跟踪

所以最大的堆栈跟踪将有大约 10000 行，平均堆栈跟踪将有大约 5000 行，所有内容都打印两次(每种异常类型一次)，总计约为 2000 * 5000 * 2 = 2000 万行。