IO中flush()函数的使用

The java.io.Writer.flush() method flushes the stream. If the stream has saved any characters from the various write() methods in a buffer, write them immediately to their intended destination. Then, if that destination is another character or byte stream, flush it. Thus one flush() invocation will flush all the buffers in a chain of Writers and OutputStreams.

public class Demo {
    public static void main(String[] ars) throws Exception {
        System.out.println("hello");
        PrintWriter writer = new PrintWriter(System.out);
        writer.println("writer start");
//      writer.flush();

        try {
            Thread.sleep();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        writer.println("writer close");
        writer.close();
    }
}
           

如上面代码，如果flush()被注释掉，则打印完“hello”之后3秒才会打印”writer start”，”writer close”，因为writer.close()在关闭输出流前会调用一次flush()。效果如下：

如果flush()没有被注释掉，则则打印完“hello”之后会立即打印”writer start”。