数据库连接池

使用数据库连接池优化程序性能

直接获取链接缺点 性能的优化

编写数据库连接池

• 编写连接池需事先java.sql.DataSource接口。DateSource接口中定义了两个重载的getConnection方法：
  Connection getConnection()
Connection getConnection(String username,String password)

• 实现DataSource接口，并实现连接池功能的步骤：

1. 在DataSource构造函数中批量创建与数据库的连接，并把创建的连接加入LinkedList对象中；
2. 实现getConnection方法，让getConnection方法每次调用时，从LinkedList中取一个Connection返回给用户；
3. 当用户使用完Connection，调用Connection.close方法时，Connection对象应保证将自己返回到LinkedList中，而不要把Conn还给数据库。

难点：Connection保证将自己返回到LinkedList中

演示

1. 建一个JdbcPool类，实现DataSource接口，里面我都有写较为详细的注释。

package cn.itcast.utils;

import javax.sql.DataSource;
import java.io.PrintWriter;
import java.sql.*;
import java.util.LinkedList;
import java.util.Map;
import java.util.Properties;
import java.util.concurrent.Executor;
import java.util.logging.Logger;

/**
 * Created by yvettee on 2017/9/28.
 */
public class JdbcPool implements DataSource {
    private static LinkedList<Connection> list = new LinkedList<Connection>();
    private static Properties prop = new Properties();

    //初始化时，要找连接池要一批链接的操作做一次就够
    static {
        try {
            prop.load(JdbcPool.class.getClassLoader().getResourceAsStream("db.properties"));
            Class.forName(prop.getProperty("driver"));

            //一上来就要10个连接
            for (int i = 0; i < 10; i++) {
                Connection conn = DriverManager.getConnection(prop.getProperty("url"), prop.getProperty("username"), prop.getProperty("password"));
                list.add(conn);
            }
        } catch (Exception e) {
            throw new ExceptionInInitializerError(e);
        }
    }


    @Override
    public Connection getConnection() throws SQLException {
        if (list.size() <= 0) {
            throw new RuntimeException("数据库忙，请稍后再来！");
        }
        //（链表）始终把第一个链接拿走
        Connection conn = list.removeFirst();
        // conn.close();这里不能直接关闭，因为会把链接还给数据库，
        // 我们的目的是从池里拿链接，也要还给池里
       MyConnection my = new MyConnection(conn);
        /*
        在实际开发，发现对象的方法满足不了开发需求时，有以下几种方法：
        1.写一个connection子类，覆盖并增强close方法（不可用）
        2.用包装设计模式（这里采用这个）
        3.用动态代理
         */
        return my;
    }

    //包装模式，1.实现与被增强对象的相同接口
    class MyConnection implements Connection {

        //2.在类中定义一个变量，记住被增强对象
        private Connection conn;

        //3.定义一个构造函数
        public MyConnection(Connection conn) {
            this.conn = conn;
        }

        //4.覆盖想增强的方法
        public void close() {
            list.add(this.conn);
        }

        @Override
        public Statement createStatement() throws SQLException {
            //5.对于不想增强的方法，直接调用目标对象（被增强对象）的方法
            return this.conn.createStatement();
        }

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

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

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

        @Override
        public void setAutoCommit(boolean autoCommit) throws SQLException {

        }

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

        @Override
        public void commit() throws SQLException {

        }

        @Override
        public void rollback() throws SQLException {

        }

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

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

        @Override
        public void setReadOnly(boolean readOnly) throws SQLException {

        }

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

        @Override
        public void setCatalog(String catalog) throws SQLException {

        }

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

        @Override
        public void setTransactionIsolation(int level) throws SQLException {

        }

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

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

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

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

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

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

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

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

        }

        @Override
        public void setHoldability(int holdability) throws SQLException {

        }

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

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

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

        @Override
        public void rollback(Savepoint savepoint) throws SQLException {

        }

        @Override
        public void releaseSavepoint(Savepoint savepoint) throws SQLException {

        }

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

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

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

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

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

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

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

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

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

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

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

        @Override
        public void setClientInfo(String name, String value) throws SQLClientInfoException {

        }

        @Override
        public void setClientInfo(Properties properties) throws SQLClientInfoException {

        }

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

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

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

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

        @Override
        public void setSchema(String schema) throws SQLException {

        }

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

        @Override
        public void abort(Executor executor) throws SQLException {

        }

        @Override
        public void setNetworkTimeout(Executor executor, int milliseconds) throws SQLException {

        }

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

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

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

    @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;
    }
}