Java的JDBC数据库连接池实现方法

Java程序员都很羡慕Windows ADO ，只需要new Connection 就可以直接从数据库连接池中返回Connection。并且 ADO Connection 是线程安全的，多个线程可以共用一个Connection，所以ASP程序一般都把getConnection 放在 Global.asa 文件中，在 IIS 启动时建立数据库连接。ADO 的Connection 和Result 都有很好的缓冲，并且很容易使用。

其实我们可以自己写一个JDBC数据库连接池。

写JDBC connection pool 的注意事项有：

1. 有一个简单的函数从连接池中得到一个 Connection。

2. close 函数必须将connection 放回 数据库连接池。

3. 当数据库连接池中没有空闲的connection，数据库连接池必须能够自动增加connection 个数。

4. 当数据库连接池中的connection 个数在某一个特别的时间变得很大，但是以后很长时间只用其中一小部分，应该可以自动将多余的connection 关闭掉。

5. 如果可能，应该提供debug 信息报告没有关闭的new Connection 。

如果要new Connection 就可以直接从数据库连接池中返回Connection， 可以这样写( Mediator pattern ) (以下代码中使用了中文全角空格)：

public class EasyConnection implements java.sql.Connection{ 
private Connection m_delegate = null; 
public EasyConnection(){ 
m_delegate = getConnectionFromPool(); 
} 
　public void close(){ 
putConnectionBackToPool(m_delegate); 
} 
public PreparedStatement prepareStatement(String sql) throws SQLException{ 
m_delegate.prepareStatement(sql); 
} 
//...... other method 
}
看来并不难。不过不建议这种写法，因为应该尽量避免使用Java Interface, 关于Java Interface 的缺点我另外再写文章讨论。大家关注的是Connection Pool 的实现方法。下面给出一种实现方法。

import java.sql.*; 
import java.lang.reflect.*; 
import java.util.*; 
import java.io.*; 

public class SimpleConnetionPool { 
private static LinkedList m_notUsedConnection = new LinkedList(); 
private static HashSet m_usedUsedConnection = new HashSet(); 
private static String m_url = ""; 
private static String m_user = ""; 
private static String m_password = ""; 
static final boolean DEBUG = true; 
static private long m_lastClearClosedConnection = System.currentTimeMillis(); 
public static long CHECK_CLOSED_CONNECTION_TIME = 4 * 60 * 60 * 1000; //4 hours 

static { 
initDriver(); 
} 

private SimpleConnetionPool() { 
} 

private static void initDriver() { 
Driver driver = null; 
//load mysql driver 
try { 
driver = (Driver) Class.forName("com.mysql.jdbc.Driver").newInstance(); 
installDriver(driver); 
} catch (Exception e) { 
} 

//load postgresql driver 
try { 
driver = (Driver) Class.forName("org.postgresql.Driver").newInstance(); 
installDriver(driver); 
} catch (Exception e) { 
} 
} 

public static void installDriver(Driver driver) { 
try { 
DriverManager.registerDriver(driver); 
} catch (Exception e) { 
e.printStackTrace(); 
} 
} 


public static synchronized Connection getConnection() { 
clearClosedConnection(); 
while (m_notUsedConnection.size() > 0) { 
try { 
ConnectionWrapper wrapper = (ConnectionWrapper) m_notUsedConnection.removeFirst(); 
if (wrapper.connection.isClosed()) { 
continue; 
} 
m_usedUsedConnection.add(wrapper); 
if (DEBUG) { 
wrapper.debugInfo = new Throwable("Connection initial statement"); 
} 
return wrapper.connection; 
} catch (Exception e) { 
} 
} 
int newCount = getIncreasingConnectionCount(); 
LinkedList list = new LinkedList(); 
ConnectionWrapper wrapper = null; 
for (int i = 0; i < newCount; i++) { 
wrapper = getNewConnection(); 
if (wrapper != null) { 
list.add(wrapper); 
} 
} 
if (list.size() == 0) { 
return null; 
} 
wrapper = (ConnectionWrapper) list.removeFirst(); 
m_usedUsedConnection.add(wrapper); 

m_notUsedConnection.addAll(list); 
list.clear(); 

return wrapper.connection; 
} 

private static ConnectionWrapper getNewConnection() { 
try { 
Connection con = DriverManager.getConnection(m_url, m_user, m_password); 
ConnectionWrapper wrapper = new ConnectionWrapper(con); 
return wrapper; 
} catch (Exception e) { 
e.printStackTrace(); 
} 
return null; 
} 

static synchronized void pushConnectionBackToPool(ConnectionWrapper con) { 
boolean exist = m_usedUsedConnection.remove(con); 
if (exist) { 
m_notUsedConnection.addLast(con); 
} 
} 

public static int close() { 
int count = 0; 

Iterator iterator = m_notUsedConnection.iterator(); 
while (iterator.hasNext()) { 
try { 
( (ConnectionWrapper) iterator.next()).close(); 
count++; 
} catch (Exception e) { 
} 
} 
m_notUsedConnection.clear(); 

iterator = m_usedUsedConnection.iterator(); 
while (iterator.hasNext()) { 
try { 
ConnectionWrapper wrapper = (ConnectionWrapper) iterator.next(); 
wrapper.close(); 
if (DEBUG) { 
wrapper.debugInfo.printStackTrace(); 
} 
count++; 
} catch (Exception e) { 
} 
} 
m_usedUsedConnection.clear(); 

return count; 
} 

private static void clearClosedConnection() { 
long time = System.currentTimeMillis(); 
//sometimes user change system time,just return 
if (time < m_lastClearClosedConnection) { 
time = m_lastClearClosedConnection; 
return; 
} 
//no need check very often 
if (time - m_lastClearClosedConnection < CHECK_CLOSED_CONNECTION_TIME) { 
return; 
} 
m_lastClearClosedConnection = time; 

//begin check 
Iterator iterator = m_notUsedConnection.iterator(); 
while (iterator.hasNext()) { 
ConnectionWrapper wrapper = (ConnectionWrapper) iterator.next(); 
try { 
if (wrapper.connection.isClosed()) { 
iterator.remove(); 
} 
} catch (Exception e) { 
iterator.remove(); 
if (DEBUG) { 
System.out.println("connection is closed, this connection initial StackTrace"); 
wrapper.debugInfo.printStackTrace(); 
} 
} 
} 

//make connection pool size smaller if too big 
int decrease = getDecreasingConnectionCount(); 
if (m_notUsedConnection.size() < decrease) { 
return; 
} 

while (decrease-- > 0) { 
ConnectionWrapper wrapper = (ConnectionWrapper) m_notUsedConnection.removeFirst(); 
try { 
wrapper.connection.close(); 
} catch (Exception e) { 
} 
} 
} 

/** 
* get increasing connection count, not just add 1 connection 
* @return count 
*/
public static int getIncreasingConnectionCount() { 
int count = 1; 
int current = getConnectionCount(); 
count = current / 4; 
if (count < 1) { 
count = 1; 
} 
return count; 
} 

/** 
* get decreasing connection count, not just remove 1 connection 
* @return count 
*/
public static int getDecreasingConnectionCount() { 
int count = 0; 
int current = getConnectionCount(); 
if (current < 10) { 
return 0; 
} 
return current / 3; 
} 

public synchronized static void printDebugMsg() { 
printDebugMsg(System.out); 
} 

public synchronized static void printDebugMsg(PrintStream out) { 
if (DEBUG == false) { 
return; 
} 
StringBuffer msg = new StringBuffer(); 
msg.append("debug message in " + SimpleConnetionPool.class.getName()); 
msg.append("\r\n"); 
msg.append("total count is connection pool: " + getConnectionCount()); 
msg.append("\r\n"); 
msg.append("not used connection count: " + getNotUsedConnectionCount()); 
msg.append("\r\n"); 
msg.append("used connection, count: " + getUsedConnectionCount()); 
out.println(msg); 
Iterator iterator = m_usedUsedConnection.iterator(); 
while (iterator.hasNext()) { 
ConnectionWrapper wrapper = (ConnectionWrapper) iterator.next(); 
wrapper.debugInfo.printStackTrace(out); 
} 
out.println(); 
} 

public static synchronized int getNotUsedConnectionCount() { 
return m_notUsedConnection.size(); 
} 

public static synchronized int getUsedConnectionCount() { 
return m_usedUsedConnection.size(); 
} 

public static synchronized int getConnectionCount() { 
return m_notUsedConnection.size() + m_usedUsedConnection.size(); 
} 

public static String getUrl() { 
return m_url; 
} 

public static void setUrl(String url) { 
if (url == null) { 
return; 
} 
m_url = url.trim(); 
} 

public static String getUser() { 
return m_user; 
} 

public static void setUser(String user) { 
if (user == null) { 
return; 
} 
m_user = user.trim(); 
} 

public static String getPassword() { 
return m_password; 
} 

public static void setPassword(String password) { 
if (password == null) { 
return; 
} 
m_password = password.trim(); 
} 

} 

class ConnectionWrapper implements InvocationHandler { 
private final static String CLOSE_METHOD_NAME = "close"; 
public Connection connection = null; 
private Connection m_originConnection = null; 
public long lastAccessTime = System.currentTimeMillis(); 
Throwable debugInfo = new Throwable("Connection initial statement"); 

ConnectionWrapper(Connection con) { 
this.connection = (Connection) Proxy.newProxyInstance( 
con.getClass().getClassLoader(), 
con.getClass().getInterfaces(), this); 
m_originConnection = con; 
} 

void close() throws SQLException { 
m_originConnection.close(); 
} 

public Object invoke(Object proxy, Method m, Object[] args) throws Throwable { 
Object obj = null; 
if (CLOSE_METHOD_NAME.equals(m.getName())) { 
SimpleConnetionPool.pushConnectionBackToPool(this); 
} 
else { 
obj = m.invoke(m_originConnection, args); 
} 
lastAccessTime = System.currentTimeMillis(); 
return obj; 
} 
}
使用方法

public class TestConnectionPool{ 
public static void main(String[] args) { 
SimpleConnetionPool.setUrl(DBTools.getDatabaseUrl()); 
SimpleConnetionPool.setUser(DBTools.getDatabaseUserName()); 
SimpleConnetionPool.setPassword(DBTools.getDatabasePassword()); 

Connection con = SimpleConnetionPool.getConnection(); 
Connection con1 = SimpleConnetionPool.getConnection(); 
Connection con2 = SimpleConnetionPool.getConnection(); 

//do something with con ... 

try { 
con.close(); 
} catch (Exception e) {} 

try { 
con1.close(); 
} catch (Exception e) {} 

try { 
con2.close(); 
} catch (Exception e) {} 

con = SimpleConnetionPool.getConnection(); 
con1 = SimpleConnetionPool.getConnection(); 
try { 
con1.close(); 
} catch (Exception e) {} 

con2 = SimpleConnetionPool.getConnection(); 
SimpleConnetionPool.printDebugMsg(); 

} 
}

转自：http://developer.51cto.com/art/200907/137300.htm