什么时候/为什么要用Java调用System.out.flush() [英] When/why to call System.out.flush() in Java

问题描述

为什么标准流不需要刷新某些流(FileOutputStream和来自套接字的流)?

Why do certain streams need to be flushed (FileOutputStream and streams from Sockets) while the standard output stream does not?

每次有人使用System.out PrintStream对象时，无论是在调用println()还是write()时，他们都不会刷新流.但是，其他程序员习惯于将其他流称为flush() a PrintStream/PrintWriter.

Every time someone uses the System.out PrintStream object, be it while calling println() or write(), they never flush the stream. However, other programmers habitually call flush() a PrintStream/PrintWriter with other streams.

我最近已经向几个程序员提出了这个问题，有些人认为Java中有一些后台处理程序可以自动刷新System.out流，但是我找不到关于它的任何文档.

I've recently asked this question to several programmers and some believe that there is some background handling in Java to auto-flush the System.out stream but I can't find any documentation on that.

这样的事情让我想知道，简单地调用System.out.println()是否与平台无关，因为某些系统可能需要您刷新流.

Something like this makes me wonder if simply calling System.out.println() is platform independent as some systems may need you to flush the stream.

推荐答案

System.out基于PrintStream，默认情况下，每当写换行符时都会刷新.

System.out is based around a PrintStream which by default flushes whenever a newline is written.

来自 javadoc :

autoFlush-一个布尔值；如果为true，则每当写入字节数组，调用println方法之一或写入换行符或字节('\n')时，都会刷新输出缓冲区.

autoFlush - A boolean; if true, the output buffer will be flushed whenever a byte array is written, one of the println methods is invoked, or a newline character or byte ('\n') is written

因此，您提到的println情况得到了明确处理，带有byte[]的write情况也保证可以刷新，因为它属于每当写入字节数组时".

So the println case you mention is explicitly handled, and the write case with a byte[] is also guaranteed to flush because it falls under "whenever a byte array is written".

如果使用

If you replace System.out using System.setOut and don't use an autoflushing stream, then you will have to flush it like any other stream.

库代码可能不应该直接使用System.out，但是如果这样做，则应该谨慎刷新，因为库用户可能会覆盖System.out来使用非刷新流.

Library code probably shouldn't be using System.out directly, but if it does, then it should be careful to flush because a library user might override System.out to use a non flushing stream.

任何将二进制输出写入System.out的Java程序都应在exit之前注意flush，因为二进制输出通常不包含尾随换行符.

Any Java program that writes binary output to System.out should be careful to flush before exit because binary output often does not include a trailing newline.

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