package JXC.Com;

import java.io.*;
import java.sql.*;
import java.util.*;
import java.util.Date;

/**
* 管理類DBConnectionManager支持對一個或多個由屬性文件定義的數據庫連接
* 池的訪問.客戶程序可以調用getInstance()方法訪問本類的唯一實例.
*/
public class DBConnectionManager {
static private DBConnectionManager instance; // 唯一實例
static private int clients;

private Vector drivers = new Vector();
private PrintWriter log;
private Hashtable pools = new Hashtable();

/**
* 返回唯一實例.如果是第一次調用此方法,則創建實例
*
* @return DBConnectionManager 唯一實例
*/
static synchronized public DBConnectionManager getInstance() {
if (instance == null) {
instance = new DBConnectionManager();
}
clients++;

return instance;
}

/**
* 構建私有函數以防止其它對象創建本類實例
*/
private DBConnectionManager() {
init();

}

/**
* 將連接對象返回給由名字指定的連接池
*
* @param name 在屬性文件中定義的連接池名字
* @param con 連接對象
*/
public void freeConnection(String name, Connection con) {
DBConnectionPool pool = (DBConnectionPool) pools.get(name);
if (pool != null) {
pool.freeConnection(con);
}
}

/**
* 獲得一個可用的(空閑的)連接.如果沒有可用連接,且已有連接數小于最大連接數
* 限制,則創建并返回新連接
*
* @param name 在屬性文件中定義的連接池名字
* @return Connection 可用連接或null
*/
public Connection getConnection(String name) {
DBConnectionPool pool = (DBConnectionPool) pools.get(name);
if (pool != null) {
return pool.getConnection();
}
return null;
}

/**
* 獲得一個可用連接.若沒有可用連接,且已有連接數小于最大連接數限制,
* 則創建并返回新連接.否則,在指定的時間內等待其它線程釋放連接.
*
* @param name 連接池名字
* @param time 以毫秒計的等待時間
* @return Connection 可用連接或null
*/
public Connection getConnection(String name, long time) {
DBConnectionPool pool = (DBConnectionPool) pools.get(name);
if (pool != null) {
return pool.getConnection(time);
}
return null;
}

/**
* 關閉所有連接,撤銷驅動程序的注冊
*/
public synchronized void release() {
// 等待直到最后一個客戶程序調用
if (--clients != 0) {
return;
}

Enumeration allPools = pools.elements();
while (allPools.hasMoreElements()) {
DBConnectionPool pool = (DBConnectionPool) allPools.nextElement();
pool.release();
}
Enumeration allDrivers = drivers.elements();
while (allDrivers.hasMoreElements()) {
Driver driver = (Driver) allDrivers.nextElement();
try {
DriverManager.deregisterDriver(driver);
log("撤銷JDBC驅動程序 " + driver.getClass().getName()+"的注冊");
}
catch (SQLException e) {
log(e, "無法撤銷下列JDBC驅動程序的注冊: " + driver.getClass().getName());
}
}
}

/**
* 根據指定屬性創建連接池實例.
*
* @param props 連接池屬性
*/
private void createPools(Properties props) {
Enumeration propNames = props.propertyNames();
while (propNames.hasMoreElements()) {
String name = (String) propNames.nextElement();
if (name.endsWith(".url")) {
String poolName = name.substring(0, name.lastIndexOf("."));
String url = props.getProperty(poolName + ".url");
if (url == null) {
log("沒有為連接池" + poolName + "指定URL");
continue;
}
String user = props.getProperty(poolName + ".user");
String password = props.getProperty(poolName + ".password");
String maxconn = props.getProperty(poolName + ".maxconn", "0");
int max;
try {
max = Integer.valueOf(maxconn).intValue();
}
catch (NumberFormatException e) {
log("錯誤的最大連接數限制: " + maxconn + " .連接池: " + poolName);
max = 0;
}
DBConnectionPool pool =
new DBConnectionPool(poolName, url, user, password, max);
pools.put(poolName, pool);
log("成功創建連接池" + poolName);
}
}
}

/**
* 讀取屬性完成初始化
*/
private void init() {
InputStream is = getClass().getResourceAsStream("/JXC/db.properties");
Properties dbProps = new Properties();
try {
dbProps.load(is);
}
catch (Exception e) {
System.err.println("不能讀取屬性文件. " +
"請確保db.properties在CLASSPATH指定的路徑中");
return;
}
String logFile = dbProps.getProperty("logfile", "DBConnectionManager.log");
try {
log = new PrintWriter(new FileWriter(logFile, true), true);
}
catch (IOException e) {
System.err.println("無法打開日志文件: " + logFile);
log = new PrintWriter(System.err);
}
loadDrivers(dbProps);
createPools(dbProps);
}

/**
* 裝載和注冊所有JDBC驅動程序
*
* @param props 屬性
*/
private void loadDrivers(Properties props) {
String driverClasses = props.getProperty("drivers");
StringTokenizer st = new StringTokenizer(driverClasses);
while (st.hasMoreElements()) {
String driverClassName = st.nextToken().trim();
try {
Driver driver = (Driver)
Class.forName(driverClassName).newInstance();
DriverManager.registerDriver(driver);
drivers.addElement(driver);
log("成功注冊JDBC驅動程序" + driverClassName);
}
catch (Exception e) {
log("無法注冊JDBC驅動程序: " +
driverClassName + ", 錯誤: " + e);
}
}
}

/**
* 將文本信息寫入日志文件
*/
private void log(String msg) {
log.println(new Date() + ": " + msg);
}

/**
* 將文本信息與異常寫入日志文件
*/
private void log(Throwable e, String msg) {
log.println(new Date() + ": " + msg);
e.printStackTrace(log);
}

/**
* 此內部類定義了一個連接池.它能夠根據要求創建新連接,直到預定的最
* 大連接數為止.在返回連接給客戶程序之前,它能夠驗證連接的有效性.
*/
class DBConnectionPool {
private int checkedOut;
private Vector freeConnections = new Vector();
private int maxConn;
private String name;
private String password;
private String URL;
private String user;

/**
* 創建新的連接池
*
* @param name 連接池名字
* @param URL 數據庫的JDBC URL
* @param user 數據庫帳號,或 null
* @param password 密碼,或 null
* @param maxConn 此連接池允許建立的最大連接數
*/
public DBConnectionPool(String name, String URL, String user, String password,
int maxConn) {
this.name = name;
this.URL = URL;
this.user = user;
this.password = password;
this.maxConn = maxConn;
}

/**
* 將不再使用的連接返回給連接池
*
* @param con 客戶程序釋放的連接
*/
public synchronized void freeConnection(Connection con) {
// 將指定連接加入到向量末尾
freeConnections.addElement(con);
checkedOut--;
notifyAll();
}

/**
* 從連接池獲得一個可用連接.如沒有空閑的連接且當前連接數小于最大連接
* 數限制,則創建新連接.如原來登記為可用的連接不再有效,則從向量刪除之,
* 然后遞歸調用自己以嘗試新的可用連接.
*/
public synchronized Connection getConnection() {
Connection con = null;
if (freeConnections.size() > 0) {
// 獲取向量中第一個可用連接
con = (Connection) freeConnections.firstElement();
freeConnections.removeElementAt(0);
try {
if (con.isClosed()) {
log("從連接池" + name+"刪除一個無效連接");
// 遞歸調用自己,嘗試再次獲取可用連接
con = getConnection();
}
}
catch (SQLException e) {
log("從連接池" + name+"刪除一個無效連接");
// 遞歸調用自己,嘗試再次獲取可用連接
con = getConnection();
}
}
else if (maxConn == 0 || checkedOut < maxConn) {
con = newConnection();
}
if (con != null) {
checkedOut++;
}
return con;
}

/**
* 從連接池獲取可用連接.可以指定客戶程序能夠等待的最長時間
* 參見前一個getConnection()方法.
*
* @param timeout 以毫秒計的等待時間限制
*/
public synchronized Connection getConnection(long timeout) {
long startTime = new Date().getTime();
Connection con;
while ((con = getConnection()) == null) {
try {
wait(timeout);
}
catch (InterruptedException e) {}
if ((new Date().getTime() - startTime) >= timeout) {
// wait()返回的原因是超時
return null;
}
}
return con;
}

/**
* 關閉所有連接
*/
public synchronized void release() {
Enumeration allConnections = freeConnections.elements();
while (allConnections.hasMoreElements()) {
Connection con = (Connection) allConnections.nextElement();
try {
con.close();
log("關閉連接池" + name+"中的一個連接");
}
catch (SQLException e) {
log(e, "無法關閉連接池" + name+"中的連接");
}
}
freeConnections.removeAllElements();
}

/**創建一個新的連接
* 創建新的連接
*/
private Connection newConnection() {
Connection con = null;
try {
if (user == null) {
con = DriverManager.getConnection(URL);
}
else {
con = DriverManager.getConnection(URL, user, password);
}
log("連接池" + name+"創建一個新的連接");
}
catch (SQLException e) {
log(e, "無法創建下列URL的連接: " + URL);
return null;
}
return con;
}
}
}