写一个数据库连接池

问题起源

在Java访问数据的时候，是使用JDBC驱动去创建数据库连接，代码如下：

 try {
    Driver mysqlDriver = (Driver) Class.forName("com.mysql.jdbc.Driver").newInstance();
    DriverManager.registerDriver(mysqlDriver);
    Connection connection = DriverManager.getConnection("jdbc:mysql://192.168.0.***:3306/rzframe?useSSL=false&serverTimezone=UTC", "root", "*******");
    Statement statement = connection.createStatement();
    ResultSet resultSet = statement.executeQuery("select * from rz_user");//查询
    connection.close();
  } catch (Exception e) {
    e.printStackTrace();
 }

我们对上面的代码做一个简单的性能测试，代码如下：

    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        CountDownLatch countDownLatch = new CountDownLatch(100);
        for (int i = 0; i < 1000; i++) {
            try {
                CountDownLatch finalCountDownLatch = countDownLatch;
                Thread thread = new Thread(() -> {
                    try {
                       doJDBC();
                    } catch (Exception ex) {

                    } finally {
                        finalCountDownLatch.countDown();
                    }
                });
                thread.start();
                if (i != 0 && i % 100 == 0) {
                    countDownLatch.await();
                    System.out.println(i);
                    countDownLatch = new CountDownLatch(100);
                }
            } catch (Exception ex) {

            }
        }
        long end = System.currentTimeMillis();
        System.out.println("耗时：" + (end - start));

    }

上面代码用了100个线程分批次去完成查询的动作，在我的机器上运行时间45s左右。

从上面的代码可以看出问题，Connection对象每一次都是重新创建，查询完成后，直接是调用close方法，如果不释放，会报连接数过多的异常。如果查询多次，那浪费在创建Connection的时间就会很多，我们知道在程序优化的手段中，有一个池化可以很好的解决这个问题。

池化的概念就是先创建多个对方存在在一个容器中，当时候的时候可以直接拿出来时候，用完后再进行归还。跟着这个思想，我们来创建自己的连接池。

编写思路

创建一个线程安全的容器（由于是多线程访问），队列或者是list，因为Connection的对象并不是有序的，所以可以使用list容器
对Connection的对象进行封装，增加一个isBusy变量，每次读取的时候就可以选出空闲的Connection对象
如果取的时候，没有可用的Connection对象，则可以再自动创建对象，可以自动扩容，直到扩容到允许的最大值。

public class PooledConnection {
    private boolean isBusy=false;
    private Connection connection;

    public PooledConnection(Connection connection, boolean b) {
        this.isBusy=b;
        this.connection=connection;
    }
    public boolean isBusy() {
        return isBusy;
    }
    public void setBusy(boolean busy) {
        isBusy = busy;
    }
    public Connection getConnection() {
        return connection;
    }
    public void setConnection(Connection connection) {
        this.connection = connection;
    }
    public void close() {
        this.setBusy(false);
    }
}

在包装好Connection后，需要考虑实现两个方法：

PooledConnection getPooledConnection();//获得一个对象

void  createPooledConnection();//创建和扩容

为了更好的程序调试，先定义几个初始的参数变量：

    //数据库相关参数
private static String jdbcDriver = null;
    private static String jdbcUrl = null;
    private static String userName = null;
    private static String password = null;
    
    //容器参数
    private static int initCount;//初始数量
    private static int stepSize;//每次扩容的数量
    private static int poolMaxSize;//最大数量
    //全局锁
     private static Lock lock;

因为有多线程访问，所以我们采用Vector集合来作为容器。

获得对象方法

1. 获得对象的方法，应该是先找到一个空闲的PooledConnection变量，如果有就直接返回。

2. 如果没有空闲的变量，则尝试进行扩充，扩充由一个线程完成，其他线程则等待，或者尝试再次获取。

 public PooledConnection getPooledConnection() throws RuntimeException, SQLException {
        PooledConnection realConnection = getRealConnection();
        while (realConnection == null) {
            if (lock.tryLock()) {//尝试获取锁
                createConnections(stepSize);//只能让一个线程扩容 获得锁之后进行扩容
                lock.unlock();
            } else {
                try {
                    Thread.sleep(200);//线程等待
                } catch (InterruptedException e) {
                }
            }
            realConnection = getRealConnection();//再次尝试获取
            if (realConnection != null) {
                return realConnection;
            }
        }
        System.out.println("线程池线程数量：" + PoolsConnections.size());
        return realConnection;
    }

private PooledConnection getRealConnection() throws SQLException {
        for (PooledConnection pooledConnection : PoolsConnections) {
            try {
                if (pooledConnection.isBusy())
                    continue;
                Connection connection = pooledConnection.getConnection();
                if (!connection.isValid(200)) {//是否有效，200ms 没有被超时
                    System.out.println("连接无效");
                    Connection validConnect = DriverManager.getConnection(jdbcUrl, userName, password);
                    pooledConnection.setConnection(validConnect);
                }
                pooledConnection.setBusy(true);
                return pooledConnection;
            } catch (SQLException e) {
                return null;
            }
        }
        return null;
    }

扩容方法对象

扩容的方法相对比较简单，判断当前对象数量有没有溢出，如果没有溢出，就进行扩容

public void createConnections(int count) throws OutofMaxCountException, IllegalArgumentException {
        if (poolMaxSize <= 0) {
            System.out.println("创建管道对象失败，最大值参数错误");
            throw new IllegalArgumentException("创建管道对象失败，最大值参数错误");
        }
        //判断是否有溢出
        boolean overFlow = isOverFlow(count);
        if (overFlow) {
            return;
        }
        System.out.println("扩容");
        for (int i = 0; i < count; i++) {
            try {
                overFlow = isOverFlow(count);
                if (overFlow)
                    return;
                Connection connection = DriverManager.getConnection(jdbcUrl, userName, password);
                PooledConnection pooledConnection = new PooledConnection(connection, false);
                PoolsConnections.add(pooledConnection);
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        System.out.println("扩容数量：" + PoolsConnections.size());
    }

    private boolean isOverFlow(int count) {
        if (PoolsConnections.size() + count >= poolMaxSize) {
            return true;
        }
        return false;
    }

上面的代码隐藏一个问题，我们增加对数据的查询方法，方便我们测试。查询方法如下：

   public ResultSet querySql(String sql) {
        try {
            PooledConnection pooledConnection = getPooledConnection();
            Connection connection = pooledConnection.getConnection();
            Statement statement = connection.createStatement();
            ResultSet resultSet = statement.executeQuery(sql);
            Thread.sleep(1000);
            pooledConnection.close();
            return resultSet;
        } catch (Exception e) {

        }
        return null;
    }

我们对代码做性能测试同样的测试，在我的电脑运行时间为5s左右，大概快了10倍。但经过多次测试，代码抛出了ConcurrentModificationException异常，这个异常的原因是因为在使用的时候，我们又修改了正在使用的对象。所以在使用的时候要对对象进行加一个读写锁。

为了锁不至于影响到锁的性能，我们把锁碎片化，采用针对每一个对象进行加锁，而不是全局加锁。修改后的封装对象：

public class PooledConnection {

    private boolean isBusy = false;
    private Connection connection;
    private ReentrantReadWriteLock reentrantReadWriteLock;

    public PooledConnection(Connection connection, boolean b) {
        this.connection = connection;
        reentrantReadWriteLock = new ReentrantReadWriteLock();
    }
    public boolean isBusy() {
        return isBusy;
    }

    public void setBusy(boolean busy) {
        isBusy = busy;
    }

    public Connection getConnection() {
        return connection;
    }
    public void setConnection(Connection connection) {
        this.connection = connection;
    }
    public void close() {
        this.setBusy(false);
    }
    public void shutDown() {
        try {
            this.connection.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

//增加读写锁的操作
    public void writeLock() {
        this.reentrantReadWriteLock.writeLock().lock();
    }
    public void unWriteLock() {
        this.reentrantReadWriteLock.writeLock().unlock();
    }
    public void readLock() {
        this.reentrantReadWriteLock.readLock().lock();
    }
    public void unReadLock() {
        this.reentrantReadWriteLock.readLock().unlock();
    }
}

最终修改后的创建和扩容，结果如下：

   public PooledConnection getPooledConnection() throws RuntimeException, SQLException {
        if (poolMaxSize <= 0) {
            System.out.println("创建管道对象失败，最大值参数错误");
            throw new IllegalArgumentException("创建管道对象失败，最大值参数错误");
        }
        PooledConnection realConnection = getRealConnection();
        while (realConnection == null) {
            if (lock.tryLock()) {//尝试获取锁
                createConnections(stepSize);//获得锁之后进行扩容
                lock.unlock();
            } else {
                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                }
            }
            realConnection = getRealConnection();
            if (realConnection != null) {
                return realConnection;
            }
        }

        return realConnection;
    }

    private PooledConnection getRealConnection() {
        for (PooledConnection pooledConnection : PoolsConnections) {
            try {
                if (pooledConnection.isBusy())
                    continue;
                /*
                此处要保证写的时候不能被读取，不然会报ConcurrentModificationException异常
                 */
                pooledConnection.writeLock();//读写互斥，写写互斥
                Connection connection = pooledConnection.getConnection();
                if (!connection.isValid(200)) {//是否有效，200ms 没有被超时
                    Connection validConnect = DriverManager.getConnection(jdbcUrl, userName, password);
                    pooledConnection.setConnection(validConnect);
                }
                pooledConnection.setBusy(true);
                pooledConnection.unWriteLock();
                return pooledConnection;
            } catch (SQLException e) {

                return null;
            }
        }
        return null;
    }
    public void createConnections(int count) throws OutofMaxCountException, IllegalArgumentException {
        if (poolMaxSize <= 0) {
            System.out.println("创建管道对象失败，最大值参数错误");
            throw new IllegalArgumentException("创建管道对象失败，最大值参数错误");
        }
        //判断是否有溢出
        boolean overFlow = isOverFlow(count);
        if (overFlow) {
            return;
        }
        System.out.println("扩容");
        for (int i = 0; i < count; i++) {
            try {
                overFlow = isOverFlow(count);
                if (overFlow)
                    return;
                Connection connection = DriverManager.getConnection(jdbcUrl, userName, password);
                PooledConnection pooledConnection = new PooledConnection(connection, false);
                PoolsConnections.add(pooledConnection);
            } catch (SQLException e) {

            }
        }
        System.out.println("扩容数量：" + PoolsConnections.size());
    }

    private boolean isOverFlow(int count) {
        if (PoolsConnections.size() + count >= poolMaxSize) {
            return true;
        }
        return false;
    }

完整地址：https://u7704756.pipipan.com/fs/7704756-387364520

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写一个数据库连接池

编写思路

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