数据库连接池的实现

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.*;
publicclass SimpleConnetionPool {
privatestatic LinkedList m_notUsedConnection = new LinkedList();
privatestatic HashSet m_usedUsedConnection = new HashSet();
privatestatic String m_url = "";
privatestatic String m_user = "";
privatestatic String m_password = "";
staticfinalboolean DEBUG = true;
staticprivatelong m_lastClearClosedConnection = System.currentTimeMillis();
publicstaticlong CHECK_CLOSED_CONNECTION_TIME = 4 * 60 * 60 * 1000; //4 hours
static {
initDriver();
}
private SimpleConnetionPool() {
}
privatestaticvoid 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) {
}
}
publicstaticvoid installDriver(Driver driver) {
try {
DriverManager.registerDriver(driver);
} catch (Exception e) {
e.printStackTrace();
}
}
publicstaticsynchronized 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) {
returnnull;
}
wrapper = (ConnectionWrapper) list.removeFirst();
m_usedUsedConnection.add(wrapper);
m_notUsedConnection.addAll(list);
list.clear();
return wrapper.connection;
}
privatestatic 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();
}
returnnull;
}
staticsynchronizedvoid pushConnectionBackToPool(ConnectionWrapper con) {
boolean exist = m_usedUsedConnection.remove(con);
if (exist) {
m_notUsedConnection.addLast(con);
}
}
publicstaticint 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;
}
privatestaticvoid 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
 */
publicstaticint 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
 */
publicstaticint getDecreasingConnectionCount() {
int count = 0;
int current = getConnectionCount();
if (current < 10) {
return0;
}
return current / 3;
}
publicsynchronizedstaticvoid printDebugMsg() {
printDebugMsg(System.out);
}
publicsynchronizedstaticvoid 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();
}
publicstaticsynchronizedint getNotUsedConnectionCount() {
return m_notUsedConnection.size();
}
publicstaticsynchronizedint getUsedConnectionCount() {
return m_usedUsedConnection.size();
}
publicstaticsynchronizedint getConnectionCount() {
return m_notUsedConnection.size() + m_usedUsedConnection.size();
}
publicstatic String getUrl() {
return m_url;
}
publicstaticvoid setUrl(String url) {
if (url == null) {
return;
}
m_url = url.trim();
}
publicstatic String getUser() {
return m_user;
}
publicstaticvoid setUser(String user) {
if (user == null) {
return;
}
m_user = user.trim();
}
publicstatic String getPassword() {
return m_password;
}
publicstaticvoid setPassword(String password) {
if (password == null) {
return;
}
m_password = password.trim();
}
}
class ConnectionWrapper implements InvocationHandler {
privatefinalstatic String CLOSE_METHOD_NAME = "close";
public Connection connection = null;
private Connection m_originConnection = null;
publiclong 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();
}
}

运行测试程序后打印JDBC数据库连接池中Connection状态，以及正在使用的没有关闭Connection信息。