先上源碼：

/*** Flushes this output stream and forces any buffered output bytes* to be written out. The general contract of <code>flush</code> is* that calling it is an indication that, if any bytes previously* written have been buffered by the implementation of the output* stream, such bytes should immediately be written to their* intended destination.* <p>* If the intended destination of this stream is an abstraction provided by* the underlying operating system, for example a file, then flushing the* stream guarantees only that bytes previously written to the stream are* passed to the operating system for writing; it does not guarantee that* they are actually written to a physical device such as a disk drive.* <p>* The <code>flush</code> method of <code>OutputStream</code> does nothing.** @exception IOException if an I/O error occurs.*/public void flush() throws IOException {}

總結有以下幾點：

1. flush()下達一條命令給緩沖區，讓它將所儲存的數據全部清空，即發送給下一級。

2. flush()刷空輸出流，并輸出所有被緩存的字節。由于某些流支持緩存功能，該方法將把緩存中所有內容強制輸出到流中。

3.?OutputStream.flush()方法將所有寫入到OutputStream的數據沖刷到相應的目標媒介中。比如，如果輸出流是FileOutputStream，那么寫入到其中的數據可能并沒有真正寫入到磁盤中。即使所有數據都寫入到了FileOutputStream，這些數據還是有可能保留在內存的緩沖區中。通過調用flush()方法，可以把緩沖區內的數據刷新到磁盤(或者網絡，以及其他任何形式的目標媒介)中。

例子：

// 模擬瀏覽器，給tomcat服務端發送符合http協議的請求消息 public static void main(String[] args) throws IOException {Socket s = new Socket("127.0.0.1", 80);PrintWriter out = new PrintWriter(s.getOutputStream());out.println("GET /myweb/test.jsp HTTP/1.1");out.println("Accept: */*");out.println("Accept-Language: zh-CN");out.println("Accept-Encoding: gzip, deflate");out.println();out.flush(); // 清空緩存并輸出流InputStream in = s.getInputStream();byte b[] = new byte[1024];int leng = 0;while((leng = in .read(b)) != -1){String str = new String(b, 0, leng);System.out.println(str);}s.close(); }

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總結

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