来自System.out.println的多线程输出是否交错 [英] Is multi-thread output from System.out.println interleaved

问题描述

如果多个线程在不同步的情况下调用System.out.println(String)，输出是否可以交错?还是每行的写入都是原子的? API 没有提到同步，所以这似乎可行，还是通过缓冲和/或VM内存模型等阻止了交错输出?

If multiple threads call System.out.println(String) without synchronization, can the output get interleaved? Or is the write of each line atomic? The API makes no mention of synchronization, so this seems possible, or is interleaved output prevented by buffering and/or the VM memory model, etc.?

例如，如果每个线程包含:

For example, if each thread contains:

System.out.println("ABC");

保证输出为:

ABC
ABC

或者是:

AABC
BC

推荐答案

由于API文档未在

Since the API documentation makes no mention of thread safety on the System.out object nor does the PrintStream#println(String) method you cannot assume that it is thread-safe.

但是，特定JVM的基础实现完全有可能对println方法使用线程安全功能(例如

However, it is entirely possible that the underlying implementation of a particular JVM uses a thread-safe function for the println method (e.g. printf on glibc) so that, in reality, the output will be guaranteed per your first example (always ABC\n then ABC\n, never interspersed characters per your second example). But keep in mind that there are lots of JVM implementations and they are only required to adhere to the JVM specification, not any conventions outside of that spec.

如果您绝对必须确保不会像您描述的那样散布println调用，那么您必须手动强制执行互斥，例如:

If you absolutely must ensure that no println calls will intersperse as you describe then you must enforce mutual exclusion manually, for example:

public void safePrintln(String s) {
  synchronized (System.out) {
    System.out.println(s);
  }
}

当然，此示例仅是示例，不应视为解决方案"；还有许多其他因素需要考虑.例如，上面的safePrintln(...)方法仅在 all 代码使用该方法并且没有任何直接调用System.out.println(...)的情况下才是安全的.

Of course, this example is only an illustration and should not be taken as a "solution"; there are many other factors to consider. For example, the safePrintln(...) method above is only safe if all code uses that method and nothing calls System.out.println(...) directly.

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