数据库连接池的概念

数据库连接背景数据库连接是一种关键的、有限的、昂贵的资源，这一点在多用户的网页应用程序中体现得尤为突出。对数据库连接的管理能显著影响到整个应用程序的伸缩性和健壮性，影响到程序的性能指标。数据库连接池正是针对这个问题提出来的。
数据库连接池数据库连接池负责分配、管理和释放数据库连接，它允许应用程序重复使用一个现有的数据库连接，而不是再重新建立一个。这项技术能明显提高对数据库操作的性能。
数据库连接池的原理没有使用数据库连接池：一个访问创建一个连接，使用完关闭连接。而频繁的创建和关闭连接非常耗时

「MySQL」自定义数据库连接池和开源数据库连接池的使用

使用数据库连接池之后：提前准备一些数据库连接，使用时从池中取出，用完归还连接池

「MySQL」自定义数据库连接池和开源数据库连接池的使用

自定义连接池

初探连接池

自定义JDBC工具类

配置文件 config.properties

driverClass=com.mysql.jdbc.Driver
url=jdbc:mysql://主机名:3306/数据库名
username=用户名
password=密码

JDBCUtils工具类

public class JDBCUtils {
    private JDBCUtils() {}  //构造函数私有化

    private static String driverClass;
    private static String url;
    private static String username;
    private static String password;
    private static Connection con;

    static {
        try {
            InputStream is = JDBCUtils.class.getClassLoader().getResourceAsStream("config.properties");
            Properties properties = new Properties();
            properties.load(is);
            driverClass = properties.getProperty("driverClass");
            url = properties.getProperty("url");
            username = properties.getProperty("username");
            password = properties.getProperty("password");
            Class.forName(driverClass);

        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    public static Connection getConnection() {  //获取连接对象
        try {
            con = DriverManager.getConnection(url, username, password);
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return con;
    }
    //关闭连接（有查询结果集）
    public static void close(Connection con, Statement stat, ResultSet res) {
        if (con != null) {
            try {
                con.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        if (stat != null) {
            try {
                stat.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        if (res != null) {
            try {
                res.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
    //关闭连接（无查询结果集）
    public static void close(Connection con, Statement stat) {
        close(con, stat, null);
    }
}

实现连接池类

定义一个连接池类并实现 java.sql.DataSource 接口。

Connection getConnection();  //获取数据库连接对象

public class MyDataSource implements DataSource{
    //定义集合容器，用于保存多个数据库连接对象
    //使用Collections 工具类实现集合的线程同步
    private static List<Connection> pool = Collections.synchronizedList(new ArrayList<Connection>());

    //静态代码块，生成10个数据库连接保存到集合中
    static {
        for (int i = 0; i < 10; i++) {
            Connection con = JDBCUtils.getConnection();
            pool.add(con);
        }
    }

    //返回连接池的大小
    public int getSize() {
        return pool.size();
    }

    //从池中返回一个数据库连接
    @Override
    public Connection getConnection() {
        if(pool.size() > 0) {
            //从池中获取数据库连接
            return pool.remove(0);
        }else {
            throw new RuntimeException("连接数量已用尽");
        }
    }

    @Override
    public Connection getConnection(String username, String password) throws SQLException {
        return null;
    }

    @Override
    public <T> T unwrap(Class<T> iface) throws SQLException {
        return null;
    }

    @Override
    public boolean isWrapperFor(Class<?> iface) throws SQLException {
        return false;
    }

    @Override
    public PrintWriter getLogWriter() throws SQLException {
        return null;
    }

    @Override
    public void setLogWriter(PrintWriter out) throws SQLException {

    }

    @Override
    public void setLoginTimeout(int seconds) throws SQLException {

    }

    @Override
    public int getLoginTimeout() throws SQLException {
        return 0;
    }

    @Override
    public Logger getParentLogger() throws SQLFeatureNotSupportedException {
        return null;
    }
}

自定义连接池的测试

public class MyDataSourceTest {
    public static void main(String[] args) throws SQLException {
        MyDataSource dataSource = new MyDataSource();
        System.out.println("使用前连接池数量：" + dataSource.getSize());

        Connection con = dataSource.getConnection();

        String sql = "select * from emp";

        PreparedStatement pst = con.prepareStatement(sql);

        ResultSet res = pst.executeQuery();
        while (res.next()) {
            String ename = res.getString("ename");
            String job = res.getString("job");
            String hiredate = res.getString("hiredate");
            System.out.println("ename:" + ename + "\t job:" + job + "\t hiredate:" + hiredate);
        }

        res.close();
        pst.close();
        con.close();
        System.out.println("使用后连接池数量：" + dataSource.getSize());
    }
}

输出：

使用前连接池数量：10
ename:SMITH	 job:CLERK	 hiredate:1980-12-17
ename:ALLEN	 job:SALESMAN	 hiredate:1981-02-20
ename:WARD	 job:SALESMAN	 hiredate:1981-02-22
ename:JONES	 job:MANAGER	 hiredate:1981-04-02
ename:MARTIN	 job:SALESMAN	 hiredate:1981-09-28
ename:BLAKE	 job:MANAGER	 hiredate:1981-05-01
ename:CLARK	 job:MANAGER	 hiredate:1981-06-09
ename:SCOTT	 job:ANALYST	 hiredate:1987-04-19
ename:KING	 job:PRESIDENT	 hiredate:1981-11-17
ename:TURNER	 job:SALESMAN	 hiredate:1981-09-08
ename:ADAMS	 job:CLERK	 hiredate:1987-05-23
ename:JAMES	 job:CLERK	 hiredate:1981-12-03
ename:FORD	 job:ANALYST	 hiredate:1981-12-03
ename:MILLER	 job:CLERK	 hiredate:1982-01-23
使用后连接池数量：9

问题：虽然我们自定义了数据库连接池，但是连接关闭以后并没有归还给数据库连接池，还需要改进归还连接的问题

继承方式改进连接池

System.out.println(JDBCUtils.getConnection());
//com.mysql.jdbc.JDBC4Connection@470e2030

通过输出 Connection 的地址发现 Connection 类的实现类是 JDBC4Connection ，是否能够通过编写一个类继承 JDBC4Connection ，然后重写 close() 方法，在关闭连接的同时归还连接？

/*
      自定义Connection类
   */
  public class MyConnection1 extends JDBC4Connection {
      //声明连接对象和连接池集合对象
      private Connection con;
      private List<Connection> pool;
  
      //通过构造方法给成员变量赋值
      public MyConnection1(String hostToConnectTo, int portToConnectTo, Properties info, String databaseToConnectTo, String url,Connection con,List<Connection> pool) throws SQLException {
          super(hostToConnectTo, portToConnectTo, info, databaseToConnectTo, url);
          this.con = con;
          this.pool = pool;
      }
  
      //重写close()方法，将连接归还给池中
      @Override
      public void close() throws SQLException {
          pool.add(con);
      }
  }

但是这种方式行不通，通过查看JDBC工具类获取连接的方法我们发现：我们虽然自定义了一个子类，完成了归还连接的操作。但是 DriverManager 获取的还是 JDBC4Connection 这个对象，并不是我们的子类对象。而我们又不能整体去修改驱动包中类的功能！

//将之前的连接对象换成自定义的子类对象
  private static MyConnection1 con;
  
  public static Connection getConnection() {
      try {
          //等效于：MyConnection1 con = new JDBC4Connection();  子类引用指向父类对象，语法错误！
          con = DriverManager.getConnection(url,username,password);
      } catch (SQLException e) {
          e.printStackTrace();
      }
  
      return con;
  }

装饰设计模式改进连接池

自定义Connection类。通过装饰设计模式，实现和mysql驱动包中的Connection实现类相同的功能！

实现步骤：

定义一个类，实现Connection接口
定义Connection连接对象和连接池容器对象的变量
提供有参构造方法，接收连接对象和连接池对象，对变量赋值
在close()方法中，完成连接的归还

剩余方法，只需要调用mysql驱动包的连接对象完成即可

public class MyConnection2 implements Connection {

  //2.定义Connection连接对象和连接池容器对象的变量
  private Connection con;
  private List<Connection> pool;

  //3.提供有参构造方法，接收连接对象和连接池对象，对变量赋值
  public MyConnection2(Connection con,List<Connection> pool) {
      this.con = con;
      this.pool = pool;
  }

  //4.在close()方法中，完成连接的归还
  @Override
  public void close() throws SQLException {
      pool.add(con);
  }


  @Override
  public Statement createStatement() throws SQLException {
      return con.createStatement();
  }

  @Override
  public PreparedStatement prepareStatement(String sql) throws SQLException {
      return con.prepareStatement(sql);
  }

  @Override
  public CallableStatement prepareCall(String sql) throws SQLException {
      return con.prepareCall(sql);
  }

  @Override
  public String nativeSQL(String sql) throws SQLException {
      return con.nativeSQL(sql);
  }

  @Override
  public void setAutoCommit(boolean autoCommit) throws SQLException {
      con.setAutoCommit(autoCommit);
  }

  @Override
  public boolean getAutoCommit() throws SQLException {
      return con.getAutoCommit();
  }

  @Override
  public void commit() throws SQLException {
      con.commit();
  }

  @Override
  public void rollback() throws SQLException {
      con.rollback();
  }

  @Override
  public boolean isClosed() throws SQLException {
      return con.isClosed();
  }

  @Override
  public DatabaseMetaData getMetaData() throws SQLException {
      return con.getMetaData();
  }

  @Override
  public void setReadOnly(boolean readOnly) throws SQLException {
      con.setReadOnly(readOnly);
  }

  @Override
  public boolean isReadOnly() throws SQLException {
      return con.isReadOnly();
  }

  @Override
  public void setCatalog(String catalog) throws SQLException {
      con.setCatalog(catalog);
  }

  @Override
  public String getCatalog() throws SQLException {
      return con.getCatalog();
  }

  @Override
  public void setTransactionIsolation(int level) throws SQLException {
      con.setTransactionIsolation(level);
  }

  @Override
  public int getTransactionIsolation() throws SQLException {
      return con.getTransactionIsolation();
  }

  @Override
  public SQLWarning getWarnings() throws SQLException {
      return con.getWarnings();
  }

  @Override
  public void clearWarnings() throws SQLException {
      con.clearWarnings();
  }

  @Override
  public Statement createStatement(int resultSetType, int resultSetConcurrency) throws SQLException {
      return con.createStatement(resultSetType,resultSetConcurrency);
  }

  @Override
  public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
      return con.prepareStatement(sql,resultSetType,resultSetConcurrency);
  }

  @Override
  public CallableStatement prepareCall(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
      return con.prepareCall(sql,resultSetType,resultSetConcurrency);
  }

  @Override
  public Map<String, Class<?>> getTypeMap() throws SQLException {
      return con.getTypeMap();
  }

  @Override
  public void setTypeMap(Map<String, Class<?>> map) throws SQLException {
      con.setTypeMap(map);
  }

  @Override
  public void setHoldability(int holdability) throws SQLException {
      con.setHoldability(holdability);
  }

  @Override
  public int getHoldability() throws SQLException {
      return con.getHoldability();
  }

  @Override
  public Savepoint setSavepoint() throws SQLException {
      return con.setSavepoint();
  }

  @Override
  public Savepoint setSavepoint(String name) throws SQLException {
      return con.setSavepoint(name);
  }

  @Override
  public void rollback(Savepoint savepoint) throws SQLException {
      con.rollback(savepoint);
  }

  @Override
  public void releaseSavepoint(Savepoint savepoint) throws SQLException {
      con.releaseSavepoint(savepoint);
  }

  @Override
  public Statement createStatement(int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
      return con.createStatement(resultSetType,resultSetConcurrency,resultSetHoldability);
  }

  @Override
  public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
      return con.prepareStatement(sql,resultSetType,resultSetConcurrency,resultSetHoldability);
  }

  @Override
  public CallableStatement prepareCall(String sql, int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
      return con.prepareCall(sql,resultSetType,resultSetConcurrency,resultSetHoldability);
  }

  @Override
  public PreparedStatement prepareStatement(String sql, int autoGeneratedKeys) throws SQLException {
      return con.prepareStatement(sql,autoGeneratedKeys);
  }

  @Override
  public PreparedStatement prepareStatement(String sql, int[] columnIndexes) throws SQLException {
      return con.prepareStatement(sql,columnIndexes);
  }

  @Override
  public PreparedStatement prepareStatement(String sql, String[] columnNames) throws SQLException {
      return con.prepareStatement(sql,columnNames);
  }

  @Override
  public Clob createClob() throws SQLException {
      return con.createClob();
  }

  @Override
  public Blob createBlob() throws SQLException {
      return con.createBlob();
  }

  @Override
  public NClob createNClob() throws SQLException {
      return con.createNClob();
  }

  @Override
  public SQLXML createSQLXML() throws SQLException {
      return con.createSQLXML();
  }

  @Override
  public boolean isValid(int timeout) throws SQLException {
      return con.isValid(timeout);
  }

  @Override
  public void setClientInfo(String name, String value) throws SQLClientInfoException {
      con.setClientInfo(name,value);
  }

  @Override
  public void setClientInfo(Properties properties) throws SQLClientInfoException {
      con.setClientInfo(properties);
  }

  @Override
  public String getClientInfo(String name) throws SQLException {
      return con.getClientInfo(name);
  }

  @Override
  public Properties getClientInfo() throws SQLException {
      return con.getClientInfo();
  }

  @Override
  public Array createArrayOf(String typeName, Object[] elements) throws SQLException {
      return con.createArrayOf(typeName,elements);
  }

  @Override
  public Struct createStruct(String typeName, Object[] attributes) throws SQLException {
      return con.createStruct(typeName,attributes);
  }

  @Override
  public void setSchema(String schema) throws SQLException {
      con.setSchema(schema);
  }

  @Override
  public String getSchema() throws SQLException {
      return con.getSchema();
  }

  @Override
  public void abort(Executor executor) throws SQLException {
      con.abort(executor);
  }

  @Override
  public void setNetworkTimeout(Executor executor, int milliseconds) throws SQLException {
      con.setNetworkTimeout(executor,milliseconds);
  }

  @Override
  public int getNetworkTimeout() throws SQLException {
      return con.getNetworkTimeout();
  }

  @Override
  public <T> T unwrap(Class<T> iface) throws SQLException {
      return con.unwrap(iface);
  }

  @Override
  public boolean isWrapperFor(Class<?> iface) throws SQLException {
      return con.isWrapperFor(iface);
  }

}

自定义连接池类

public class MyDataSource implements DataSource{
      //定义集合容器，用于保存多个数据库连接对象
      private static List<Connection> pool = Collections.synchronizedList(new ArrayList<Connection>());
  
      //静态代码块，生成10个数据库连接保存到集合中
      static {
          for (int i = 0; i < 10; i++) {
              Connection con = JDBCUtils.getConnection();
              pool.add(con);
          }
      }
  
      //返回连接池的大小
      public int getSize() {
          return pool.size();
      }
  
      //从池中返回一个数据库连接
      @Override
      public Connection getConnection() {
          if(pool.size() > 0) {
              //从池中获取数据库连接
              Connection con = pool.remove(0);
              //通过自定义连接对象进行包装
              MyConnection2 mycon = new MyConnection2(con,pool);
              //返回包装后的连接对象
              return mycon;
          }else {
              throw new RuntimeException("连接数量已用尽");
          }
      }
  }

缺点： Connection 接口中要实现的方法太多了，代码繁杂

适配器设计模式改进连接池

提供一个适配器类，实现 Connection 接口，将所有功能进行实现(除了 close()方法)，作为中间类。自定义连接类只需要继承这个适配器类，重写需要改进的 close() 方法即可！

适配器类不需要实现 close() 方法，所以定义为抽象类

public abstract class MyAdapter implements Connection {
  
      // 定义数据库连接对象的变量
      private Connection con;
  
      // 通过构造方法赋值
      public MyAdapter(Connection con) {
          this.con = con;
      }
  
      // 所有的方法，均调用mysql的连接对象实现
      @Override
      public Statement createStatement() throws SQLException {
          return con.createStatement();
      }
  
      @Override
      public PreparedStatement prepareStatement(String sql) throws SQLException {
          return con.prepareStatement(sql);
      }
  
      @Override
      public CallableStatement prepareCall(String sql) throws SQLException {
          return con.prepareCall(sql);
      }
  
      @Override
      public String nativeSQL(String sql) throws SQLException {
          return con.nativeSQL(sql);
      }
  
      @Override
      public void setAutoCommit(boolean autoCommit) throws SQLException {
          con.setAutoCommit(autoCommit);
      }
  
      @Override
      public boolean getAutoCommit() throws SQLException {
          return con.getAutoCommit();
      }
  
      @Override
      public void commit() throws SQLException {
          con.commit();
      }
  
      @Override
      public void rollback() throws SQLException {
          con.rollback();
      }
  
      @Override
      public boolean isClosed() throws SQLException {
          return con.isClosed();
      }
  
      @Override
      public DatabaseMetaData getMetaData() throws SQLException {
          return con.getMetaData();
      }
  
      @Override
      public void setReadOnly(boolean readOnly) throws SQLException {
          con.setReadOnly(readOnly);
      }
  
      @Override
      public boolean isReadOnly() throws SQLException {
          return con.isReadOnly();
      }
  
      @Override
      public void setCatalog(String catalog) throws SQLException {
          con.setCatalog(catalog);
      }
  
      @Override
      public String getCatalog() throws SQLException {
          return con.getCatalog();
      }
  
      @Override
      public void setTransactionIsolation(int level) throws SQLException {
          con.setTransactionIsolation(level);
      }
  
      @Override
      public int getTransactionIsolation() throws SQLException {
          return con.getTransactionIsolation();
      }
  
      @Override
      public SQLWarning getWarnings() throws SQLException {
          return con.getWarnings();
      }
  
      @Override
      public void clearWarnings() throws SQLException {
          con.clearWarnings();
      }
  
      @Override
      public Statement createStatement(int resultSetType, int resultSetConcurrency) throws SQLException {
          return con.createStatement(resultSetType,resultSetConcurrency);
      }
  
      @Override
      public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
          return con.prepareStatement(sql,resultSetType,resultSetConcurrency);
      }
  
      @Override
      public CallableStatement prepareCall(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
          return con.prepareCall(sql,resultSetType,resultSetConcurrency);
      }
  
      @Override
      public Map<String, Class<?>> getTypeMap() throws SQLException {
          return con.getTypeMap();
      }
  
      @Override
      public void setTypeMap(Map<String, Class<?>> map) throws SQLException {
          con.setTypeMap(map);
      }
  
      @Override
      public void setHoldability(int holdability) throws SQLException {
          con.setHoldability(holdability);
      }
  
      @Override
      public int getHoldability() throws SQLException {
          return con.getHoldability();
      }
  
      @Override
      public Savepoint setSavepoint() throws SQLException {
          return con.setSavepoint();
      }
  
      @Override
      public Savepoint setSavepoint(String name) throws SQLException {
          return con.setSavepoint(name);
      }
  
      @Override
      public void rollback(Savepoint savepoint) throws SQLException {
          con.rollback(savepoint);
      }
  
      @Override
      public void releaseSavepoint(Savepoint savepoint) throws SQLException {
          con.releaseSavepoint(savepoint);
      }
  
      @Override
      public Statement createStatement(int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
          return con.createStatement(resultSetType,resultSetConcurrency,resultSetHoldability);
      }
  
      @Override
      public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
          return con.prepareStatement(sql,resultSetType,resultSetConcurrency,resultSetHoldability);
      }
  
      @Override
      public CallableStatement prepareCall(String sql, int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
          return con.prepareCall(sql,resultSetType,resultSetConcurrency,resultSetHoldability);
      }
  
      @Override
      public PreparedStatement prepareStatement(String sql, int autoGeneratedKeys) throws SQLException {
          return con.prepareStatement(sql,autoGeneratedKeys);
      }
  
      @Override
      public PreparedStatement prepareStatement(String sql, int[] columnIndexes) throws SQLException {
          return con.prepareStatement(sql,columnIndexes);
      }
  
      @Override
      public PreparedStatement prepareStatement(String sql, String[] columnNames) throws SQLException {
          return con.prepareStatement(sql,columnNames);
      }
  
      @Override
      public Clob createClob() throws SQLException {
          return con.createClob();
      }
  
      @Override
      public Blob createBlob() throws SQLException {
          return con.createBlob();
      }
  
      @Override
      public NClob createNClob() throws SQLException {
          return con.createNClob();
      }
  
      @Override
      public SQLXML createSQLXML() throws SQLException {
          return con.createSQLXML();
      }
  
      @Override
      public boolean isValid(int timeout) throws SQLException {
          return con.isValid(timeout);
      }
  
      @Override
      public void setClientInfo(String name, String value) throws SQLClientInfoException {
          con.setClientInfo(name,value);
      }
  
      @Override
      public void setClientInfo(Properties properties) throws SQLClientInfoException {
          con.setClientInfo(properties);
      }
  
      @Override
      public String getClientInfo(String name) throws SQLException {
          return con.getClientInfo(name);
      }
  
      @Override
      public Properties getClientInfo() throws SQLException {
          return con.getClientInfo();
      }
  
      @Override
      public Array createArrayOf(String typeName, Object[] elements) throws SQLException {
          return con.createArrayOf(typeName,elements);
      }
  
      @Override
      public Struct createStruct(String typeName, Object[] attributes) throws SQLException {
          return con.createStruct(typeName,attributes);
      }
  
      @Override
      public void setSchema(String schema) throws SQLException {
          con.setSchema(schema);
      }
  
      @Override
      public String getSchema() throws SQLException {
          return con.getSchema();
      }
  
      @Override
      public void abort(Executor executor) throws SQLException {
          con.abort(executor);
      }
  
      @Override
      public void setNetworkTimeout(Executor executor, int milliseconds) throws SQLException {
          con.setNetworkTimeout(executor,milliseconds);
      }
  
      @Override
      public int getNetworkTimeout() throws SQLException {
          return con.getNetworkTimeout();
      }
  
      @Override
      public <T> T unwrap(Class<T> iface) throws SQLException {
          return con.unwrap(iface);
      }
  
      @Override
      public boolean isWrapperFor(Class<?> iface) throws SQLException {
          return con.isWrapperFor(iface);
      }
  }

自定义连接类

通过适配器设计模式。完成close()方法的重写

定义一个类，继承适配器父类
定义 Connection 连接对象和连接池容器对象的变量
提供有参构造方法，接收连接对象和连接池对象，对变量赋值

在 close() 方法中，完成连接的归还

public class MyConnection3 extends MyAdapter { //2.定义Connection连接对象和连接池容器对象的变量 private Connection con; private List pool;

  //3.提供有参构造方法，接收连接对象和连接池对象，对变量赋值
  public MyConnection3(Connection con,List<Connection> pool) {
      super(con);    // 将接收的数据库连接对象给适配器父类传递
      this.con = con;
      this.pool = pool;
  }

  //4.在close()方法中，完成连接的归还
  @Override
  public void close() throws SQLException {
      pool.add(con);
  }

}

自定义连接池类

public class MyDataSource implements DataSource{
      //定义集合容器，用于保存多个数据库连接对象
      private static List<Connection> pool = Collections.synchronizedList(new ArrayList<Connection>());
  
      //静态代码块，生成10个数据库连接保存到集合中
      static {
          for (int i = 0; i < 10; i++) {
              Connection con = JDBCUtils.getConnection();
              pool.add(con);
          }
      }
  
      //返回连接池的大小
      public int getSize() {
          return pool.size();
      }
  
      //从池中返回一个数据库连接
      @Override
      public Connection getConnection() {
          if(pool.size() > 0) {
              //从池中获取数据库连接
              Connection con = pool.remove(0);
  
              //通过自定义连接对象进行包装
              MyConnection3 mycon = new MyConnection3(con,pool);
  
              //返回包装后的连接对象
              return mycon;
          }else {
              throw new RuntimeException("连接数量已用尽");
          }
      }
  }

缺点：自定义连接类中的方法已经很简洁了。剩余所有的方法已经抽取到了适配器类中。但是适配器这个类还是我们自己编写的，也比较麻烦！所以可以使用动态代理的方式来改进。

动态代理

public class MyDataSource implements DataSource{
      //定义集合容器，用于保存多个数据库连接对象
      private static List<Connection> pool = Collections.synchronizedList(new ArrayList<Connection>());
  
      //静态代码块，生成10个数据库连接保存到集合中
      static {
          for (int i = 0; i < 10; i++) {
              Connection con = JDBCUtils.getConnection();
              pool.add(con);
          }
      }
  
      //返回连接池的大小
      public int getSize() {
          return pool.size();
      }
  
      //动态代理方式
      @Override
      public Connection getConnection() {
          if(pool.size() > 0) {
              //从池中获取数据库连接
              Connection con = pool.remove(0);
  
              Connection proxyCon = (Connection)Proxy.newProxyInstance(con.getClass().getClassLoader(), new Class[]{Connection.class}, new InvocationHandler() {
                  /*
                      执行Connection实现类所有方法都会经过invoke
                      如果是close方法，则将连接还回池中
                      如果不是，直接执行实现类的原有方法
                   */
                  @Override
                  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                      if(method.getName().equals("close")) {
                          pool.add(con);
                          return null;
                      }else {
                          return method.invoke(con,args);
                      }
                  }
              });
  
              return proxyCon;
          }else {
              throw new RuntimeException("连接数量已用尽");
          }
      }
  
  
      //从池中返回一个数据库连接
      /*@Override
      public Connection getConnection() {
          if(pool.size() > 0) {
              //从池中获取数据库连接
              Connection con = pool.remove(0);
  
              //通过自定义连接对象进行包装
              //MyConnection2 mycon = new MyConnection2(con,pool);
              MyConnection3 mycon = new MyConnection3(con,pool);
  
              //返回包装后的连接对象
              return mycon;
          }else {
              throw new RuntimeException("连接数量已用尽");
          }
      }*/
  }

开源连接池的使用

C3P0连接池

导入jar包
导入配置文件到src目录下
创建c3p0连接池对象
获取数据库连接进行使用

配置文件 c3p0-config.xml 注意该配置文件的名字是固定的不要改，否则无法识别

initialPoolSize ：初始化连接数量
maxPoolSize ：最大连接数量，当连接数量超过初始化连接数量时，会在连接池内继续创建连接，直到达到数据库连接池所能容纳的最大连接数量

checkoutTimeout ：超过时间。如果使用的连接数量超过最大连接数量，编译器会在 checkoutTimeout 时间以后报错并终止程序。

com.mysql.jdbc.Driver jdbc:mysql://主机名:3306/数据库名用户名密码

<!-- 连接池参数 -->
<!-- 初始化连接数量 -->
<property name="initialPoolSize">5</property>
<!--  最大连接数量  -->
<property name="maxPoolSize">10</property>
<!--  超时时间  -->
<property name="checkoutTimeout">3000</property>

com.mysql.jdbc.Driver jdbc:mysql://主机名:3306/数据库名用户名密码

<!-- 连接池参数 -->
<property name="initialPoolSize">5</property>
<property name="maxPoolSize">8</property>
<property name="checkoutTimeout">1000</property>

C3P0数据库连接池的使用

public static void main(String[] args) throws SQLException {
        //创建c3p0连接池对象
        DataSource dataSource = new ComboPooledDataSource();
        //获取数据库连接进行使用
        Connection con = dataSource.getConnection();
        String s = "select *from emp";
        PreparedStatement pst = con.prepareStatement(s);
        ResultSet rs = pst.executeQuery();
        while (rs.next()) {
            String ename = rs.getString("ename");
            String job = rs.getString("job");
            String hiredate = rs.getString("hiredate");
            System.out.println("ename:" + ename + " job:" + job + " hiredate:" + hiredate);
        }
        rs.close();
        pst.close();
        con.close(); // 将连接对象归还池中
    }

Druid连接池

导入jar包
编写配置文件，放在src目录下
通过Properties集合加载配置文件
通过Druid连接池工厂类获取数据库连接池对象
获取数据库连接，进行使用

配置文件 druid.properties

driverClassName=com.mysql.jdbc.Driver
url=jdbc:mysql://主机名:3306/数据库名
username:用户名
password:密码
# 初始连接数量
initialSize=5
# 最大连接数量
maxActive=10
# 最长等待时间
maxWait=3000

Druid数据库的使用

public static void main(String[] args) throws Exception {
        //通过Properties集合加载配置文件
        InputStream is = demo01.class.getClassLoader().getResourceAsStream("druid.properties");
        Properties prop = new Properties();
        prop.load(is);
        //通过Druid连接池工厂类获取数据库连接池对象
        DataSource dataSource = DruidDataSourceFactory.createDataSource(prop);
        //获取数据库连接，进行使用
        Connection con = dataSource.getConnection();
        PreparedStatement pst = con.prepareStatement("select *from emp");
        ResultSet rs = pst.executeQuery();
        while (rs.next()) {
            String ename = rs.getString("ename");
            String job = rs.getString("job");
            String hiredate = rs.getString("hiredate");
            System.out.println("ename:" + ename + " job:" + job + " hiredate:" + hiredate);
        }
        rs.close();
        pst.close();
        con.close();
    }

抽取工具类

/*
      数据库连接池工具类
   */
  public class DataSourceUtils {
      //1.私有构造方法
      private DataSourceUtils(){}
  
      //2.定义DataSource数据源变量
      private static DataSource dataSource;
  
      //3.提供静态代码块，完成配置文件的加载和获取连接池对象
      static {
          try{
              //加载配置文件
              InputStream is = DruidDemo1.class.getClassLoader().getResourceAsStream("druid.properties");
              Properties prop = new Properties();
              prop.load(is);
  
              //获取数据库连接池对象
              dataSource = DruidDataSourceFactory.createDataSource(prop);
  
          } catch(Exception e) {
              e.printStackTrace();
          }
      }
  
      //4.提供获取数据库连接的方法
      public static Connection getConnection() {
          Connection con = null;
          try {
              con = dataSource.getConnection();
          } catch (SQLException e) {
              e.printStackTrace();
          }
          return con;
      }
  
      //5.提供获取数据库连接池的方法
      public static DataSource getDataSource() {
          return dataSource;
      }
  
      //6.提供释放资源的方法
      public static void close(Connection con, Statement stat, ResultSet rs) {
          if(con != null) {
              try {
                  con.close();
              } catch (SQLException e) {
                  e.printStackTrace();
              }
          }
  
          if(stat != null) {
              try {
                  stat.close();
              } catch (SQLException e) {
                  e.printStackTrace();
              }
          }
  
          if(rs != null) {
              try {
                  rs.close();
              } catch (SQLException e) {
                  e.printStackTrace();
              }
          }
      }
  
      public static void close(Connection con, Statement stat) {
          close(con,stat,null);
      }
  
  }

工具类的使用

public static void main(String[] args) throws SQLException {
        //利用工具类获取DataSoure
        DataSource dataSource = DataSourceUtils.getDataSource();
        //获取连接，并使用
        Connection con = dataSource.getConnection();
        String s = "select *from emp";
        PreparedStatement pst = con.prepareStatement(s);
        ResultSet rs = pst.executeQuery();
        while (rs.next()) {
            String ename = rs.getString("ename");
            String job = rs.getString("job");
            String hiredate = rs.getString("hiredate");
            System.out.println("ename:" + ename + " job:" + job + " hiredate:" + hiredate);
        }
        DataSourceUtils.close(con, pst, rs);
    }