Java基礎: PrintWriter

希望您在看這篇部落格之前，知道Java IO技術，至少寫過IO操作的Demo！

另外建議您看看： Java IO：IO流中的flush方法

這篇部落格，主要使用Socket執行個體（很簡單的小例子）來引出問題，然後在探討問題。

1. Socket服務端

package mark.zhang;import java.io.BufferedReader;import java.io.InputStream;import java.io.InputStreamReader;import java.net.ServerSocket;import java.net.Socket;public class Server {public static void main(String[] args) throws Exception {ServerSocket server = new ServerSocket(3000);Socket socket = server.accept();InputStream is = socket.getInputStream();BufferedReader br = new BufferedReader(new InputStreamReader(is));String info = br.readLine();System.out.println("server receive info: " + info);}}

2. Socket用戶端

package mark.zhang;import java.io.OutputStream;import java.io.PrintWriter;import java.net.Socket;public class Client {public static void main(String[] args) throws Exception {Socket client = new Socket("127.0.0.1", 3000);OutputStream os = client.getOutputStream();PrintWriter pw = new PrintWriter(os, true);pw.print("Hello,Server");pw.close();// pw.println("Hello,Server");}}

代碼很簡單，主要是用戶端向服務端發送一個字串，服務端將接收到的資訊列印出來。

上面代碼，運行（先運行Server.java後運行Client.java）之後，一切正常！

經過Debug，我發現，調用層次：

PrintWriter的 close()方法 ---> BufferedWriter的close()方法 ---> OutputStreamWriter的close()方法 ---> StreamEncoder的close()方法

BufferedWriter的close()方法源碼：

public void close() throws IOException {synchronized (lock) {    if (out == null) {return;    }    try {        flushBuffer();    } finally {        out.close();        out = null;        cb = null;    }}

OutputStreamWriter的close()方法源碼：

public void close() throws IOException {se.close();}

StreamEncoder的close()方法源碼可以參看http://blog.csdn.net/androidbluetooth/article/details/6460726這篇文章的代碼。

那麼，我們做點調整，改變這些正常反應吧！修改用戶端代碼（注釋掉pw.close()）：

package mark.zhang;import java.io.OutputStream;import java.io.PrintWriter;import java.net.Socket;public class Client {public static void main(String[] args) throws Exception {Socket client = new Socket("127.0.0.1", 3000);OutputStream os = client.getOutputStream();PrintWriter pw = new PrintWriter(os, true);pw.print("Hello,Server");// pw.close();// pw.println("Hello,Server");}}

服務端列印結果：

server receive info: null

也就是說，沒有接收到任何資訊。也就是說，這種方式在服務端無法接收到資訊。

惡作劇一把之後，我們再次修改代碼，如下：

package mark.zhang;import java.io.OutputStream;import java.io.PrintWriter;import java.net.Socket;public class Client {public static void main(String[] args) throws Exception {Socket client = new Socket("127.0.0.1", 3000);OutputStream os = client.getOutputStream();PrintWriter pw = new PrintWriter(os, true);//pw.print("Hello,Server");//pw.close();pw.println("Hello,Server");}}

ok，服務端可以接收到資訊。看看PrintWriter類的println方法源碼：

public void println(String x) {synchronized (lock) {    print(x);    println();}

public void print(String s) {if (s == null) {    s = "null";}write(s);}

public void println() {newLine();}

newLine()就是加入一個斷行符號符：

private void newLine() {try {    synchronized (lock) {ensureOpen();out.write(lineSeparator);if (autoFlush)    out.flush();    }}catch (InterruptedIOException x) {    Thread.currentThread().interrupt();}catch (IOException x) {    trouble = true;}    }

lineSeparator定義：

lineSeparator = (String) java.security.AccessController.doPrivileged(               new sun.security.action.GetPropertyAction("line.separator"));

服務端的readLine()方法，遇到斷行符號符就讀一次資料（一行一行的讀）。所以服務端可以收到資訊。

3. 研究一下readLine

再次修改Client.java代碼：

package mark.zhang;import java.io.DataOutputStream;import java.io.OutputStream;import java.net.Socket;public class Client {public static void main(String[] args) throws Exception {Socket client = new Socket("127.0.0.1", 3000);OutputStream os = client.getOutputStream();DataOutputStream dos = new DataOutputStream(os);dos.writeBytes("Hello,Server1");dos.writeBytes("\n");dos.writeBytes("Hello,Server2");//dos.close();}}

服務端接收資訊：

server receive info: Hello,Server1

說明，沒有接收到Hello,Server2。這就說明，readLine遇到斷行符號就算讀完資訊。

關於readLine()方法的其它用法，見下代碼：

package csdn.zhwx;import java.io.BufferedReader;import java.io.BufferedWriter;import java.io.File;import java.io.FileInputStream;import java.io.FileOutputStream;import java.io.IOException;import java.io.InputStreamReader;import java.io.OutputStreamWriter;import java.nio.charset.Charset;public class BufferedReaderTest {public static void getFile(String readFilePath, String readFileName,String writeFilePath, String writeFileName) {File reaFile = null;FileInputStream fis = null;BufferedReader bReader = null;File writeFile = null;FileOutputStream fos = null;BufferedWriter bWriter = null;try {if (!(readFileName == null || readFileName.length() <= 0)) {reaFile = new File(readFilePath + readFileName);fis = new FileInputStream(reaFile);// bReader = new BufferedReader(new InputStreamReader(fis,"GBK"));//指定GBK編碼，一般用在漢化windows上。// bReader = new BufferedReader(new InputStreamReader(fis));//預設編碼，和系統編碼一致。linux為UTF-8bReader = new BufferedReader(new InputStreamReader(fis, "UTF-8"));// 指定UTF-8編碼writeFile = new File(writeFilePath, writeFileName);fos = new FileOutputStream(writeFile);// bWriter = new BufferedWriter(new OutputStreamWriter(fos));//預設編碼，和系統編碼一致。linux為UTF-8bWriter = new BufferedWriter(new OutputStreamWriter(fos,"UTF-8"));// 指定UTF-8編碼String str = null;while ((str = bReader.readLine()) != null) {// 該方法不會將讀取到的換行\n寫到檔案中// System.out.println(str);// 要自己手動加\nbWriter.write(str + "\n");/*bWriter.write(str);bWriter.newLine();*/// 不重新整理的話，檔案不會有內容bWriter.flush();}}} catch (Exception e) {e.printStackTrace();try {fis.close();bReader.close();fos.close();bWriter.close();} catch (IOException e1) {e1.printStackTrace();}} finally {try {fis.close();bReader.close();fos.close();bWriter.close();} catch (IOException e1) {e1.printStackTrace();}}}public static void main(String[] args) {getFile("/home/zhihui/Downloads/", "hello.txt","/home/zhihui/Downloads/", "copyFormHello.txt");//linux下測試/*當前JRE：1.6.0_23當前JVM的預設字元集：UTF-8*/System.out.println("當前JRE：" + System.getProperty("java.version"));        System.out.println("當前JVM的預設字元集：" + Charset.defaultCharset()); }}