C3P0多数据源的死锁问题

最近在写的数据迁移工具完成的差不多了，今天将连接池换成C3P0，发现一个问题，就是配置了多个数据源的C3P0在同时获取不同数据源的Connection时会发生死锁。 1.运行如下的代码，用JProfiler测试，会发现死锁的情况： 代码： package com.highgo.test.c3p0dea

最近在写的数据迁移工具完成的差不多了，今天将连接池换成C3P0，发现一个问题，就是配置了多个数据源的C3P0在同时获取不同数据源的Connection时会发生死锁。

1.运行如下的代码，用JProfiler测试，会发现死锁的情况：

代码：

package com.highgo.test.c3p0deadlock;

import java.sql.SQLException;

import com.mchange.v2.c3p0.ComboPooledDataSource;

//加锁source个postgre的ComboPooledDataSource的getConnection用一个锁
public class Test {

	public static void main(String[] args) throws InterruptedException {
		ComboPooledDataSource source = new ComboPooledDataSource("source");
		ComboPooledDataSource source2 = new ComboPooledDataSource("source");
		ComboPooledDataSource postgres = new ComboPooledDataSource("postgres");
		ComboPooledDataSource postgres2 = new ComboPooledDataSource("postgres");
		new Thread(new SourceGetConn(source), "source").start();
//		new Thread(new SourceGetConn(source2), "source2").start();
//		Thread.sleep(1000);
		new Thread(new DestGetConn(postgres), "postgres").start();
//		new Thread(new DestGetConn(postgres2), "postgres2").start();
	}

}

class SourceGetConn implements Runnable {

	private ComboPooledDataSource source = null;

	public SourceGetConn(ComboPooledDataSource source) {
		this.source = source;
	}

	@Override
	public void run() {
		while (true) {
			try {
				Thread.sleep(1000);
				source.getConnection();
				System.out.println("I get a Connection! I am in " + Thread.currentThread().getName());
			} catch (InterruptedException | SQLException e) {
				e.printStackTrace();
			}
		}
	}

}

class DestGetConn implements Runnable {

	private ComboPooledDataSource postgres = null;

	public DestGetConn(ComboPooledDataSource source) {
		this.postgres = source;
	}

	@Override
	public void run() {
		while (true) {
			try {
				Thread.sleep(1000);
				postgres.getConnection();
				System.out.println("I get a Connection! I am in " + Thread.currentThread().getName());
			} catch (InterruptedException | SQLException e) {
				e.printStackTrace();
			}
		}
	}

}

死锁情况：

可以看到source和postgre两个进程都被一个没有记录的对象锁住了。

2.将上边的代码的Thread.sleep注释去掉，在运行，是不会有死锁问题的，于是查看C3P0的源代码，ComboPooledDataSource@getConnection是继承自AbstractPoolBackedDataSource#getConnection，代码如下：

public Connection getConnection() throws SQLException
    {
        PooledConnection pc = getPoolManager().getPool().checkoutPooledConnection();
        return pc.getConnection();
    }

    public Connection getConnection(String username, String password) throws SQLException
    { 
        PooledConnection pc = getPoolManager().getPool(username, password).checkoutPooledConnection();
        return pc.getConnection();
    }

先看这个PoolManager，AbstractPoolBackedDataSource#getPoolManager方法的实现如下，是线程安全的

 private synchronized C3P0PooledConnectionPoolManager getPoolManager() throws SQLException
    {
        if (poolManager == null)
        {
            ConnectionPoolDataSource cpds = assertCpds();
            poolManager = new C3P0PooledConnectionPoolManager(cpds, null, null, this.getNumHelperThreads(), this.getIdentityToken(), this.getDataSourceName());
            if (logger.isLoggable(MLevel.INFO))
                logger.info("Initializing c3p0 pool... " + this.toString( true )  /* + "; using pool manager: " + poolManager */);
        }
        return poolManager;	    
    }

从上边的代码也可以看出，一个DataSource实例，只保持一个PoolManager的引用。
再接着看getPool方法，也是线程安全的；

public synchronized C3P0PooledConnectionPool getPool(String username, String password, boolean create) throws SQLException
    {
        if (create)
            return getPool( username, password );
        else
        {
            DbAuth checkAuth = new DbAuth( username, password );
            C3P0PooledConnectionPool out = (C3P0PooledConnectionPool) authsToPools.get(checkAuth);
            if (out == null)
                throw new SQLException("No pool has been initialized for databse user '" + username + "' with the specified password.");
            else
                return out;
        }
    }

再看C3P0PooledConnectionPool#checkoutPooledConnection（）；

public PooledConnection checkoutPooledConnection() throws SQLException
    { 
        //System.err.println(this + " -- CHECKOUT");
        try 
	    { 
		PooledConnection pc = (PooledConnection) this.checkoutAndMarkConnectionInUse(); 
		pc.addConnectionEventListener( cl );
		return pc;
	    }
        catch (TimeoutException e)
        { throw SqlUtils.toSQLException("An attempt by a client to checkout a Connection has timed out.", e); }
        catch (CannotAcquireResourceException e)
        { throw SqlUtils.toSQLException("Connections could not be acquired from the underlying database!", "08001", e); }
        catch (Exception e)
        { throw SqlUtils.toSQLException(e); }
    }

返回一个C3P0PooledConnection 实例；C3P0PooledConnection 这个类里的方法都是线程安全的。ComboPooledDataSource@getConnection的最后一站就是C3P0PooledConnection#getConnection；如下：

 public synchronized Connection getConnection()
	throws SQLException
    { 
	if ( exposedProxy != null)
	    {
		//DEBUG
		//System.err.println("[DOUBLE_GET_TESTER] -- double getting a Connection from " + this );
		//new Exception("[DOUBLE_GET_TESTER] -- Double-Get Stack Trace").printStackTrace();
		//origGet.printStackTrace();

// 		System.err.println("c3p0 -- Uh oh... getConnection() was called on a PooledConnection when " +
// 				   "it had already provided a client with a Connection that has not yet been " +
// 				   "closed. This probably indicates a bug in the connection pool!!!");

		logger.warning("c3p0 -- Uh oh... getConnection() was called on a PooledConnection when " +
			       "it had already provided a client with a Connection that has not yet been " +
			       "closed. This probably indicates a bug in the connection pool!!!");

		return exposedProxy;
	    }
	else
	    { return getCreateNewConnection(); }
    }

从上边的源码分析可以看出，一个ComboPooledDataSource实例的ComboPooledDataSource@getConnection是线程安全的，可以放心调用；可以测试一下，将最开始的代码稍微修改下，如下：

package com.highgo.test.c3p0deadlock;

import java.sql.SQLException;

import com.mchange.v2.c3p0.ComboPooledDataSource;


//加锁source个postgre的ComboPooledDataSource的getConnection用一个锁
public class Test {

	public static void main(String[] args) throws InterruptedException {
		ComboPooledDataSource source = new ComboPooledDataSource("source");
//		ComboPooledDataSource source2 = new ComboPooledDataSource("source");
		ComboPooledDataSource postgres = new ComboPooledDataSource("postgres");
//		ComboPooledDataSource postgres2 = new ComboPooledDataSource("postgres");
		new Thread(new SourceGetConn(source), "source").start();
		new Thread(new SourceGetConn(source), "source2").start();
//		Thread.sleep(1000);
//		new Thread(new DestGetConn(postgres), "postgres").start();
//		new Thread(new DestGetConn(postgres2), "postgres2").start();
	}

}

class SourceGetConn implements Runnable {

	private ComboPooledDataSource source = null;

	public SourceGetConn(ComboPooledDataSource source) {
		this.source = source;
	}

	@Override
	public void run() {
		while (true) {
			try {
				Thread.sleep(1000);
				source.getConnection();
				System.out.println("I get a Connection! I am in " + Thread.currentThread().getName());
			} catch (InterruptedException | SQLException e) {
				e.printStackTrace();
			}
		}
	}

}

class DestGetConn implements Runnable {

	private ComboPooledDataSource postgres = null;

	public DestGetConn(ComboPooledDataSource source) {
		this.postgres = source;
	}

	@Override
	public void run() {
		while (true) {
			try {
				Thread.sleep(1000);
				postgres.getConnection();
				System.out.println("I get a Connection! I am in " + Thread.currentThread().getName());
			} catch (InterruptedException | SQLException e) {
				e.printStackTrace();
			}
		}
	}

}

将一个ComboPooledDataSource实例，传给两个线程分别getConnection，getConnection的过程没有加锁的情况下是可以运行的，完全没有问题。
3.经过测试发现同一个数据源的两个ComboPooledDataSource实例，getConnection方法不加锁的情况下，也是没有问题的。

稍微总结一下：

C3P0在一个ComboPooledDataSource实例的getConnection方法是线程安全的

C3P0在一个数据源的多个ComboPooledDataSource实例的getConnection方法也是线程安全的

C3P0在多个数据源的多个ComboPooledDataSource不同时调用getConnection的情况下，不会发生死锁（基于概率，若干时间之后，肯定会发生死锁）

C3P0在多个数据源的多个ComboPooledDataSource实例的getConnection方法同时（相邻的两行代码）调用时，会发生死锁现象，如1中所述

4.总结：

属于不同数据源的多个ComboPooledDataSource实例的getConnection方法调用要互斥

测试代码如下：

package com.highgo.test.c3p0deadlock;

import java.sql.SQLException;
import java.util.concurrent.locks.ReentrantLock;

import com.mchange.v2.c3p0.ComboPooledDataSource;

//加锁source个postgre的ComboPooledDataSource的getConnection用一个锁
public class Test2 {

	public static void main(String[] args) throws InterruptedException {
		ComboPooledDataSource source = new ComboPooledDataSource("source");
		ComboPooledDataSource source2 = new ComboPooledDataSource("source");
		ComboPooledDataSource postgres = new ComboPooledDataSource("postgres");
		ComboPooledDataSource postgres2 = new ComboPooledDataSource("postgres");
		ReentrantLock lock = new ReentrantLock();
		new Thread(new SourceGetConn2(source, lock), "source").start();
		new Thread(new SourceGetConn2(source2, lock), "source2").start();
		Thread.sleep(1000);
		new Thread(new DestGetConn2(postgres, lock), "postgres").start();
		new Thread(new DestGetConn2(postgres2, lock), "postgres2").start();
	}

}

class SourceGetConn2 implements Runnable {

	private ComboPooledDataSource source = null;
	private ReentrantLock lock;

	public SourceGetConn2(ComboPooledDataSource source, ReentrantLock lock) {
		this.source = source;
		this.lock = lock;
	}

	@Override
	public void run() {
		while (true) {
			try {
				Thread.sleep(1000);
				lock.lock();
				source.getConnection();
				lock.unlock();
				System.out.println("I get a Connection! I am in " + Thread.currentThread().getName());
			} catch (InterruptedException | SQLException e) {
				e.printStackTrace();
			}
		}
	}

}

class DestGetConn2 implements Runnable {

	private ComboPooledDataSource postgres = null;
	private ReentrantLock lock;

	public DestGetConn2(ComboPooledDataSource source, ReentrantLock lock) {
		this.postgres = source;
		this.lock = lock;
	}

	@Override
	public void run() {
		while (true) {
			try {
				Thread.sleep(1000);
				lock.lock();
				postgres.getConnection();
				lock.unlock();
				System.out.println("I get a Connection! I am in " + Thread.currentThread().getName());
			} catch (InterruptedException | SQLException e) {
				e.printStackTrace();
			}
		}
	}

}

5.最后总结一个效率还可以的工具类

package com.highgo.hgdbadmin.myutil;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import com.mchange.v2.c3p0.ComboPooledDataSource;

public class C3P0Util {

	public static String SOURCE = "source";
	public static String POSTGRES = "postgres";

	private ComboPooledDataSource source = null;
	private ComboPooledDataSource postgres = null;

	private static C3P0Util instance = null;

	private C3P0Util() {
		source = new ComboPooledDataSource("source");
		postgres = new ComboPooledDataSource("postgres");
	}

	public static final synchronized C3P0Util getInstance() {
		if (instance == null) {
			instance = new C3P0Util();
		}
		return instance;
	}

	public synchronized Connection getConnection(String dataSource) throws SQLException {
		if ("source".equals(dataSource)) {
			return source.getConnection();
		} else if ("postgres".equals(dataSource)) {
			return postgres.getConnection();
		}
		return null;
	}

	public synchronized void close(Connection conn) {
		try {
			if (conn != null) {
				conn.close();
				conn = null;
			}
		} catch (SQLException e) {
		}
	}

	public synchronized void close(Statement stat) {
		try {
			if (stat != null) {
				stat.close();
				stat = null;
			}
		} catch (SQLException e) {
		}
	}

	public synchronized void close(ResultSet rest) {
		try {
			if (rest != null) {
				rest.close();
				rest = null;
			}
		} catch (SQLException e) {
		}
	}

	public static void main(String[] args) {
		new Thread(new TestThread(), "test").start();
	}

	private static class TestThread implements Runnable {

		private String dataSource = "source";

		@Override
		public void run() {
			while (true) {
				try {
					Connection conn = C3P0Util.getInstance().getConnection("");
					System.out.println("hello,this is " + dataSource);
				} catch (SQLException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				if ("source".equals(dataSource)) {
					dataSource = "postgres";
				} else {
					dataSource = "source";
				}
			}

		}

	}
}