奇怪的'out'变量，System.out.println（） [英] strange 'out' variable, System.out.println()

问题描述

以下是来自java.lang.System类的代码（JDK版本1.6）

Following is the code from java.lang.System class (JDK version 1.6)

public final static PrintStream out = nullPrintStream(); //out is set to 'null'

private static PrintStream nullPrintStream() throws NullPointerException {
    if (currentTimeMillis() > 0) {
        return null;
    }
    throw new NullPointerException();
}

当我们写 System.out.println（在我们的代码中有些东西）; 那么为什么即使'out'设置为'null'也不会得到NullPointerException

when we write System.out.println("Something"); in our code then why don't we get NullPointerException even when 'out' is set to 'null'

无论如何 out 将通过系统类中的 setOut 方法设置

Anyhow out will be set via following setOut method in System class

public static void setOut(PrintStream out) {
     checkIO();
     setOut0(out);
 }

为什么JLS需要 nullPrintStream 方法？

Theyn why JLS needs nullPrintStream method ?

推荐答案

看一下 private static void initializeSystemClass（） - 调用此方法启动，它调用 setOut0（）这是一个 native 方法。这将 Stream 绑定到它应该的位置。

Take a look at the private static void initializeSystemClass() - this method is called to start things up, it calls setOut0() which is a native method. This ties the Stream into where it's supposed to be.

所以即使该字段可能看起来 public static final 它实际上不是， native 代码更改它。

So even though the field may look public static final it actually isn't, the native code changes it.

编辑

OP问那么为什么JLS需要nullPrintStream方法？

这与java编译器有关 - 它将内联静态最终字段（如果它们被分配给常量）在编译时，如 null 。编译器实际上会用常量替换每个对该字段的引用。

This is to do with the java compiler - it will "inline" static final fields if they are assigned to something constant at compile time, like null. The compiler will actually replace each reference to the field with the constant.

这会破坏初始化，因为对象将不再持有对的引用流但是 null 。将流分配给方法的返回会阻止内联。

This would break the initialisation as objects would no longer hold a reference to the Stream but to null. Assigning the stream to the return of a method prevents the inlining.

有些人可能称之为脏黑客。错误引用俾斯麦JDK就像香肠一样，最好不要看它的制作。

Some might call it a dirty hack. To misquote Bismarck "The JDK is like sausages, it's best not to see it being made".

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