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(3000);
		}
		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”。

总结

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