BlogJava-Avenue U-随笔分类-Core Java

JDBC: why 'Class.forName' at first

JosephQuinn — Sun, 14 Jun 2009 16:44:00 GMT

First, here is the following basic code:

1 package com.googlesites.qslbinghamton.corejava.database;
2
3 import java.sql.Connection;
4 import java.sql.DriverManager;
5 import java.sql.ResultSet;
6 import java.sql.SQLException;
7 import java.sql.Statement;
8
9 public class MySqlConn {
10     public static Connection getConnection() throws Exception {
11         String driver = "com.mysql.jdbc.Driver";
12         String url = "jdbc:mysql://localhost/t2";
13         String username = "root";
14         String password = "12345678";
15
16         Class.forName(driver);
17         Connection conn = DriverManager.getConnection(url, username, password);
18         return conn;
19     }
20
21     public static void main(String[] args) {
22         Connection conn = null;
23         Statement stmt = null;
24         ResultSet rs = null;
25         try {
26             conn = getConnection();
27             System.out.println("conn=" + conn);
28             // prepare query
29             String query = "select * from Employee";
30             // create a statement
31             stmt = conn.createStatement();
32             // execute query and return result as a ResultSet
33             rs = stmt.executeQuery(query);
34             // extract data from the ResultSet
35             while (rs.next()) {
36                 String id = rs.getString(1);
37                 String username = rs.getString(2);
38
39                 System.out.println("id=" + id);
40                 System.out.println("name=" + username);
41                 System.out.println("---------------");
42             }
43         } catch (Exception e) {
44             e.printStackTrace();
45             System.exit(1);
46         } finally {
47             // release database resources
48             try {
49                 rs.close();
50                 stmt.close();
51                 conn.close();
52             } catch (SQLException e) {
53                 e.printStackTrace();
54             }
55         }
56     }
57 }
58

Here is a trivial question, why Class.forName in the first place.
Now, let's take a look at class com.mysql.jdbc.Driver source file:

1 package com.mysql.jdbc;
2
3 import java.sql.SQLException;
4
5 /**
6  * The Java SQL framework allows for multiple database drivers. Each driver
7  * should supply a class that implements the Driver interface
8  *
9  *

10  * The DriverManager will try to load as many drivers as it can find and then
11  * for any given connection request, it will ask each driver in turn to try to
12  * connect to the target URL.
13  *
14  *

15  * It is strongly recommended that each Driver class should be small and
16  * standalone so that the Driver class can be loaded and queried without
17  * bringing in vast quantities of supporting code.
18  *
19  *

20  * When a Driver class is loaded, it should create an instance of itself and
21  * register it with the DriverManager. This means that a user can load and
22  * register a driver by doing Class.forName("foo.bah.Driver")
23  *
24  * @see org.gjt.mm.mysql.Connection
25  * @see java.sql.Driver
26  * @author Mark Matthews
27  * @version $Id$
28  */
29 public class Driver extends NonRegisteringDriver implements java.sql.Driver {
30     // ~ Static fields/initializers
31     // ---------------------------------------------
32
33     //
34     // Register ourselves with the DriverManager
35     //
36     static {
37         try {
38             java.sql.DriverManager.registerDriver(new Driver());
39         } catch (SQLException E) {
40             throw new RuntimeException("Can't register driver!");
41         }
42     }
43
44     // ~ Constructors
45     // -----------------------------------------------------------
46
47     /**
48      * Construct a new driver and register it with DriverManager
49      *
50      * @throws SQLException
51      *             if a database error occurs.
52      */
53     public Driver() throws SQLException {
54         // Required for Class.forName().newInstance()
55     }
56 }
57

Take a close look in class Driver's static block:

java.sql.DriverManager.registerDriver(new Driver());

It ensures the new created Driver object is registered every time after Driver class is created or Driver instance is created.

Class.forName(driver); make sure the code in static block run at first which means a new Driver class object is created during the compile time.

For better understanding, here is a sample program which have a static block in the class

class A {
    static {
        System.out.println("Class A loaded");
    }

    public A() {
        System.out.println("create a instance of A");
    }
}

public class Main {

    public static void main(String[] args) throws Exception {
        Class.forName("com.javaye.A");
    }
}

The output from class Main is

Class A loaded

Now, change the above code a little bit in public static void main function: only initiate class A a=null.

1 class A {
2     static {
3         System.out.println("Class A loaded");
4     }
5
6     public A() {
7         System.out.println("create a instance of A");
8     }
9 }
10
11 public class Main {
12
13     public static void main(String[] args) throws Exception {
14         A a = null;
15
16     }
17 }

There is no output at this time. This is because only with a variable name 'a' doesn't change the fact that no new object from class 'A' is created and the static block doesn't
run neither.

Change the code again:

The outputs are:
Class A loaded
create a instance of A
create a instance of A

Clearly, with new A(), static block only runs once(The basic concept of static) and constructor of A runs twice as 2 objects are creates.

JosephQuinn 2009-06-15 00:44 发表评论

A Socket Code Sample in Java

JosephQuinn — Sat, 13 Jun 2009 02:30:00 GMT

It's an implementation that can sent requests from multi-clients to server, meanwhile, the server handle the requests by multi-thread.

1. Server side code:

1 package com.googlesites.qslbinghamton.corejava.socket;
2
3 import java.io.BufferedReader;
4 import java.io.IOException;
5 import java.io.InputStream;
6 import java.io.InputStreamReader;
7 import java.io.OutputStream;
8 import java.io.PrintWriter;
9 import java.net.*;
10 import java.util.concurrent.*;
11
12 public class MultiThreadServer {
13     private int port=8821;
14     private ServerSocket serverSocket;
15     private ExecutorService executorService;//线程池
16     private final int POOL_SIZE=10;//单个CPU线程池大小
17
18     public MultiThreadServer() throws IOException{
19         serverSocket=new ServerSocket(port);
20         //Runtime的availableProcessor()方法返回当前系统的CPU数目.
21         executorService=Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors()*POOL_SIZE);
22         System.out.println("服务器启动");
23     }
24
25     public void service(){
26         while(true){
27             Socket socket=null;
28             try {
29                 //接收客户连接,只要客户进行了连接,就会触发accept();从而建立连接
30                 socket=serverSocket.accept();
31                 executorService.execute(new Handler(socket));
32
33             } catch (Exception e) {
34                 e.printStackTrace();
35             }
36         }
37     }
38
39     public static void main(String[] args) throws IOException {
40         new MultiThreadServer().service();
41     }
42
43 }
44
45 class Handler implements Runnable{
46     private Socket socket;
47     public Handler(Socket socket){
48         this.socket=socket;
49     }
50     private PrintWriter getWriter(Socket socket) throws IOException{
51         OutputStream socketOut=socket.getOutputStream();
52         return new PrintWriter(socketOut,true);
53     }
54     private BufferedReader getReader(Socket socket) throws IOException{
55         InputStream socketIn=socket.getInputStream();
56         return new BufferedReader(new InputStreamReader(socketIn));
57     }
58     public String echo(String msg){
59         return "echo:"+msg;
60     }
61     public void run(){
62         try {
63             System.out.println("New connection accepted "+socket.getInetAddress()+":"+socket.getPort());
64             BufferedReader br=getReader(socket);
65             PrintWriter pw=getWriter(socket);
66             String msg=null;
67             while((msg=br.readLine())!=null){
68                 System.out.println(msg);
69                 pw.println(echo(msg));
70                 if(msg.equals("bye"))
71                     break;
72             }
73         } catch (IOException e) {
74             e.printStackTrace();
75         }finally{
76             try {
77                 if(socket!=null)
78                     socket.close();
79             } catch (IOException e) {
80                 e.printStackTrace();
81             }
82         }
83     }
84 }

2. Client side code

1 package com.googlesites.qslbinghamton.corejava.socket;
2
3 import java.io.BufferedReader;
4 import java.io.IOException;
5 import java.io.InputStreamReader;
6 import java.io.OutputStream;
7 import java.net.Socket;
8 import java.util.concurrent.ExecutorService;
9 import java.util.concurrent.Executors;
10
11 public class MultiThreadClient {
12
13     public static void main(String[] args) {
14         int numTasks = 10;
15
16         ExecutorService exec = Executors.newCachedThreadPool();
17
18         for (int i = 0; i < numTasks; i++) {
19             exec.execute(createTask(i));
20         }
21
22     }
23
24     // 定义一个简单的任务
25     private static Runnable createTask(final int taskID) {
26         return new Runnable() {
27             private Socket socket = null;
28             private int port=8821;
29
30             public void run() {
31                 System.out.println("Task " + taskID + ":start");
32                 try {
33                     socket = new Socket("localhost", port);
34                     // 发送关闭命令
35                     OutputStream socketOut = socket.getOutputStream();
36                     socketOut.write("shutdown\r\n".getBytes());
37
38                     // 接收服务器的反馈
39                     BufferedReader br = new BufferedReader(
40                             new InputStreamReader(socket.getInputStream()));
41                     String msg = null;
42                     while ((msg = br.readLine()) != null)
43                         System.out.println(msg);
44                 } catch (IOException e) {
45                     e.printStackTrace();
46                 }
47             }
48
49         };
50     }
51 }
52

JosephQuinn 2009-06-13 10:30 发表评论