java Eclipse bug？仅使用默认大小写打开空值

Question

我在试验enum，发现以下代码在Eclipse上编译并运行良好（构建id:20090920-1017，不确定确切的编译器版本）：

public class SwitchingOnAnull {
    enum X { ,; }
    public static void main(String[] args) {
        X x = null;
        switch(x) {
            default: System.out.println("Hello world!");
        }
    }
}

在使用Eclipse编译和运行时，它会打印"Hello world!"并正常退出

对于javac编译器，这会像预期的那样抛出NullPointerException

那么Eclipse Java编译器中是否存在缺陷

Answer 1

这是一个错误。下面是根据Java语言规范第三版为switch语句指定的行为：

JLS 14.11 The ^{} Statement

SwitchStatement:
    switch ( Expression ) SwitchBlock

When the switch statement is executed, first the Expression is evaluated. If the Expression evaluates to null, a NullPointerException is thrown and the entire switch statement completes abruptly for that reason.

显然，Eclipse中的错误与default案例或enum完全无关

public class SwitchingOnAnull {
    public static void main(String[] args) {        
        java.math.RoundingMode x = null;
        switch(x) {};

        switch((Integer) null) {};

        switch((Character) null) {
            default: System.out.println("I've got sunshine!");
        }       
    }
}

以上代码在Eclipse（至少在某些版本）上编译并运行“良好”。当使用javac编译时，每个switch抛出一个NullPointerException，这正是规范要求的

---

原因

以下是在Eclipse下编译时的javap -c SwitchingOnAnull：

Compiled from "SwitchingOnAnull.java"
public class SwitchingOnAnull extends java.lang.Object{
public SwitchingOnAnull();
Code:
 0: aload_0
 1: invokespecial  #8; //Method java/lang/Object."<init>":()V
 4: return

public static void main(java.lang.String[]);
Code:
 0: aconst_null
 1: astore_1
 2: getstatic     #16; //Field java/lang/System.out:Ljava/io/PrintStream;
 5: ldc           #22; //String I've got sunshine!
 7: invokevirtual #24; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
10: return

}

Eclipse编译器似乎完全摆脱了整个switch结构。不幸的是，这种优化破坏了语言规范

---

官方用语

该漏洞已被归档并分配修复

Olivier Thomann 2010-05-28 08:37:21 EDT

We are too aggressive on the optimization.

For:

  switch((Integer) null) {};

we optimize out the whole switch statement when we should at least evaluate the expression.

I'll take a look.

Candidate for 3.6.1.

另见

• Bug 314830 - Switching on a ^{} expression doesn't always throw ^{}

Answer 2

是的。根据JLS，这是一个错误：

If the switch expression is of a reference type, such as a boxed primitive type or an enum, a run-time error will occur if the expression evaluates to null at run-time.

Answer 3

当然。如果我们看一下java语言规范的chapter 14.11，它清楚地指出（在“讨论”下）：

The prohibition against using null as a switch label prevents one from writing code that can never be executed. If the switch expression is of a reference type, such as a boxed primitive type or an enum, a run-time error will occur if the expression evaluates to null at run-time.