package ch03.section03;

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

//該類的作用：連接管理數據庫緩沖池


/**
 * 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();

  /**
   * 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--;
      log("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++;
        log("con=" + 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;
    }
  }

  /**
   * 建構函數私有以防止其它對象創建本類實例
   */
  private DBConnectionManager() {
    init();
  }

  /**
   * 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);
      }
    }
  }

  /**
   * 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;
  }

  /**
   * 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;
  }

  /**
   * 讀取屬性完成初始化
   */
  private void init() {
    String dbpath = "/db.properties";
    InputStream is = getClass().getResourceAsStream(dbpath);
    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);
  }

  /**
   * 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());
      }
    }
  }
}