使用方法引用而不是lambda表达式抛出java.lang.NullPointerException [英] java.lang.NullPointerException is thrown using a method-reference but not a lambda expression

问题描述

我注意到使用Java 8方法引用的未处理异常有些奇怪。这是我的代码，使用lambda表达式（） - > s.toLowerCase（）：

I've noticed something weird about unhandled exceptions using Java 8 method reference. This is my code, using the lambda expression () -> s.toLowerCase():

public class Test {

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

    private static void testNPE(String s) {
        Thread t = new Thread(() -> s.toLowerCase());
//        Thread t = new Thread(s::toLowerCase);
        t.setUncaughtExceptionHandler((t1, e) -> System.out.println("Exception!"));
        t.start();
    }
}

它打印Exception，所以它工作正常。但是，当我更改 Thread t 以使用方法引用时（甚至IntelliJ建议）：

It prints "Exception", so it works fine. But when I change Thread t to use a method-reference (even IntelliJ suggests that):

Thread t = new Thread(s::toLowerCase);

未捕获异常：

Exception in thread "main" java.lang.NullPointerException
    at Test.testNPE(Test.java:9)
    at Test.main(Test.java:4)
    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:497)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

有人能解释一下这里发生了什么吗？

Can someone explain what is going on here?

推荐答案

此行为依赖于方法引用和lambda表达式的评估过程之间的细微差别。

This behaviour relies on a subtle difference between the evaluation process of method-references and lambda expressions.

来自JLS 方法参考的运行时评估：

首先，如果方法引用表达式以ExpressionName或Primary开头，则计算此子表达式。 如果子表达式求值为 null ，则会引发 NullPointerException ，并且方法引用表达式突然完成。

First, if the method reference expression begins with an ExpressionName or a Primary, this subexpression is evaluated. If the subexpression evaluates to null, a NullPointerException is raised, and the method reference expression completes abruptly.

使用以下代码：

Thread t = new Thread(s::toLowerCase); // <-- s is null, NullPointerException thrown here
t.setUncaughtExceptionHandler((t1, e) -> System.out.println("Exception!"));

表达式 s 评估为 null 并且在评估该方法引用时会完全抛出异常。但是，那时没有附加异常处理程序，因为此代码将在执行之后执行。

the expression s is evaluated to null and an exception is thrown exactly when that method-reference is evaluated. However, at that time, no exception handler was attached, since this code would be executed after.

在lambda表达式的情况下不会发生这种情况，因为lambda将在不执行其正文的情况下进行评估。来自 Lambda的运行时评估表达式：

This doesn't happen in the case of a lambda expression, because the lambda will be evaluated without its body being executed. From Run-Time Evaluation of Lambda Expressions:

评估lambda表达式与执行lambda体不同。

Evaluation of a lambda expression is distinct from execution of the lambda body.

Thread t = new Thread(() -> s.toLowerCase());
t.setUncaughtExceptionHandler((t1, e) -> System.out.println("Exception!"));

即使 s 是 null ，将正确创建lambda表达式。然后将附加异常处理程序，线程将启动，抛出异常，将由处理程序捕获。

Even if s is null, the lambda expression will be correctly created. Then the exception handler will be attached, the thread will start, throwing an exception, that will be caught by the handler.

作为旁注，Eclipse Mars.2似乎有一个小错误：即使使用方法引用，它也会调用异常处理程序。 Eclipse不应该在 s :: toLowerCase 中抛出 NullPointerException ，因此应该在以后延迟异常，添加了异常处理程序。

As a side-note, it seems Eclipse Mars.2 has a small bug regarding this: even with the method-reference, it invokes the exception handler. Eclipse isn't throwing a NullPointerException at s::toLowerCase when it should, thus deferring the exception later on, when the exception handler was added.

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