PrintWriter应用实例

希望您在看这篇博客之前，知道Java IO技术，至少写过IO操作的Demo！

另外建议您看看： Java IO：IO流中的flush方法

这篇博客，主要使用Socket实例（很简单的小例子）来引出问题，然后在探讨问题。

1. Socket服务端

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);
    }

}

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客户端

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");
    }
}

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");
	}
}

代码很简单，主要是客户端向服务端发送一个字符串，服务端将接收到的信息打印出来。

上面代码，运行（先运行Server.java后运行Client.java）之后，一切正常！

经过Debug，我发现，调用层次：

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

BufferedWriter的close()方法源码：

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

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

OutputStreamWriter的close()方法源码：

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

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

StreamEncoder的close()方法源码可以参看 http://blog.csdn.net/androidbluetooth/article/details/6460726这篇文章的代码。

那么，我们做点调整，改变这些正常反应吧！修改客户端代码（注释掉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");
    }
}

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");
	}
}

服务端打印结果：

server receive info: null

server receive info: null

也就是说，没有接收到任何信息。也就是说，这种方式在服务端无法接收到信息。

恶作剧一把之后，我们再次修改代码，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");
    }
}

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，服务端可以接收到信息。看看PrintWriter类的println方法源码：

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

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

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

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

public void println() {
    newLine();
}

public void println() {
	newLine();
}

newLine()就是加入一个回车符：

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;
    }
    }

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定义：

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

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

服务端的readLine()方法，遇到回车符就读一次数据（一行一行的读）。所以服务端可以收到信息。

3. 研究一下readLine

再次修改Client.java代码：

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();
    }
}

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();
	}
}

服务端接收信息：

server receive info: Hello,Server1

server receive info: Hello,Server1

说明，没有接收到Hello,Server2。这就说明，readLine遇到回车就算读完信息。

关于readLine()方法的其它用法，见下代码：

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编码，一般用在汉化windows上。
                // bReader = new BufferedReader(new InputStreamReader(fis));//默认编码，和系统编码一致。linux为UTF-8
                bReader = new BufferedReader(
                        new InputStreamReader(fis, "UTF-8"));// 指定UTF-8编码

                writeFile = new File(writeFilePath, writeFileName);
                fos = new FileOutputStream(writeFile);
                // bWriter = new BufferedWriter(new OutputStreamWriter(fos));//默认编码，和系统编码一致。linux为UTF-8
                bWriter = new BufferedWriter(new OutputStreamWriter(fos,
                        "UTF-8"));// 指定UTF-8编码

                String str = null;
                while ((str = bReader.readLine()) != null) {// 该方法不会将读取到的换行\n写到文件中
                    // System.out.println(str);
                    // 要自己手动加\n
                    bWriter.write(str + "\n");

                    /*bWriter.write(str);
                    bWriter.newLine();*/

                    // 不刷新的话，文件不会有内容
                    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下测试
        /*当前JRE：1.6.0_23
        当前JVM的默认字符集：UTF-8*/
        System.out.println("当前JRE：" + System.getProperty("java.version"));
        System.out.println("当前JVM的默认字符集：" + Charset.defaultCharset());
    }
}