连接池-Mybatis源码
持续更新:https://github.com/dchack/Mybatis-source-code-learn
Mybatis连接池
有这么个定律,有连接的地方就有池。
在市面上,可以适配Mybatis DateSource的连接池有很对,比如:
Mybatis也自带来连接池的功能,先学习下Mybatis的,相对简单的实现。
涉及的类:
PoolState
public class PoolState {
protected PooledDataSource dataSource;
// 空闲连接集合
protected final List<PooledConnection> idleConnections = new ArrayList<PooledConnection>();
// 正在使用的连接集合
protected final List<PooledConnection> activeConnections = new ArrayList<PooledConnection>();
// 请求次数,每次获取连接,都会自增,用于
protected long requestCount = 0;
// 累计请求耗时,每次获取连接时计算累加,除以requestCount可以获得平均耗时
protected long accumulatedRequestTime = 0;
// 累计连接使用时间
protected long accumulatedCheckoutTime = 0;
// 过期连接次数
protected long claimedOverdueConnectionCount = 0;
protected long accumulatedCheckoutTimeOfOverdueConnections = 0;
// 累计等待获取连接时间
protected long accumulatedWaitTime = 0;
// 等待获取连接的次数
protected long hadToWaitCount = 0;
// 连接已关闭的次数
protected long badConnectionCount = 0;
public PoolState(PooledDataSource dataSource) {
this.dataSource = dataSource;
}
public synchronized long getRequestCount() {
return requestCount;
}
public synchronized long getAverageRequestTime() {
return requestCount == 0 ? 0 : accumulatedRequestTime / requestCount;
}
public synchronized long getAverageWaitTime() {
return hadToWaitCount == 0 ? 0 : accumulatedWaitTime / hadToWaitCount;
}
public synchronized long getHadToWaitCount() {
return hadToWaitCount;
}
public synchronized long getBadConnectionCount() {
return badConnectionCount;
}
public synchronized long getClaimedOverdueConnectionCount() {
return claimedOverdueConnectionCount;
}
public synchronized long getAverageOverdueCheckoutTime() {
return claimedOverdueConnectionCount == 0 ? 0 : accumulatedCheckoutTimeOfOverdueConnections / claimedOverdueConnectionCount;
}
public synchronized long getAverageCheckoutTime() {
return requestCount == 0 ? 0 : accumulatedCheckoutTime / requestCount;
}
public synchronized int getIdleConnectionCount() {
return idleConnections.size();
}
public synchronized int getActiveConnectionCount() {
return activeConnections.size();
}
}
注意代码中的字段都是用protected修饰的,表示pooled包内都可访问,在写这份代码的时候必然默认这个包下实现一个独立的功能,内部字段都可以共享使用,否则都写set,get方法太麻烦了。
在PoolState
类中,很多指标比如requestCount
,claimedOverdueConnectionCount
等都不和连接池核心逻辑相关,纯粹只是表示连接池的一些指标而已。
作为连接池,在这里最重要的就是两个List:
- idleConnections
- activeConnections
这两个都是ArrayList,所以在整个实现中我们是通过synchronized
关键字来处理并发场景的。
PooledConnection
组成池的两个List中存储的是PooledConnection
,而PooledConnection
通过java动态代理机制实现代理真正Connection。
PooledConnection
继承InvocationHandler
,所以实现了invoke
方法:
/*
* Required for InvocationHandler implementation.
*
* @param proxy - not used
* @param method - the method to be executed
* @param args - the parameters to be passed to the method
* @see java.lang.reflect.InvocationHandler#invoke(Object, java.lang.reflect.Method, Object[])
*/
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
String methodName = method.getName();
if (CLOSE.hashCode() == methodName.hashCode() && CLOSE.equals(methodName)) {
dataSource.pushConnection(this);
return null;
} else {
try {
if (!Object.class.equals(method.getDeclaringClass())) {
// issue #579 toString() should never fail
// throw an SQLException instead of a Runtime
checkConnection();
}
return method.invoke(realConnection, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
}
private void checkConnection() throws SQLException {
if (!valid) {
throw new SQLException("Error accessing PooledConnection. Connection is invalid.");
}
}
主要看到这个代理实现处理了close
方法,就是将连接从使用列表中弹出。
对于其他方法,会判断方法是否属于Object中的方法,如果不是则进行连接合法的校验,然后执行真正Connection
即realConnection
中对应的方法。
获得一个代理类的代码,即调用Proxy.newProxyInstance
方法,在PooledConnection
中的构造函数中:
/*
* Constructor for SimplePooledConnection that uses the Connection and PooledDataSource passed in
*
* @param connection - the connection that is to be presented as a pooled connection
* @param dataSource - the dataSource that the connection is from
*/
public PooledConnection(Connection connection, PooledDataSource dataSource) {
this.hashCode = connection.hashCode();
this.realConnection = connection;
this.dataSource = dataSource;
this.createdTimestamp = System.currentTimeMillis();
this.lastUsedTimestamp = System.currentTimeMillis();
this.valid = true;
this.proxyConnection = (Connection) Proxy.newProxyInstance(Connection.class.getClassLoader(), IFACES, this);
}
我们可以看到realConnection
是在构造函数时就传入的了。
而配置这个池的参数都是在PooledDataSource
中:
官方文档:
poolMaximumActiveConnections – 在任意时间可以存在的活动(也就是正在使用)连接数量,默认值:10
poolMaximumIdleConnections – 任意时间可能存在的空闲连接数。
poolMaximumCheckoutTime – 在被强制返回之前,池中连接被检出(checked out)时间,默认值:20000 毫秒(即 20 秒)
poolTimeToWait – 这是一个底层设置,如果获取连接花费了相当长的时间,连接池会打印状态日志并重新尝试获取一个连接(避免在误配置的情况下一直安静的失败),默认值:20000 毫秒(即 20 秒)。
poolMaximumLocalBadConnectionTolerance – 这是一个关于坏连接容忍度的底层设置, 作用于每一个尝试从缓存池获取连接的线程。 如果这个线程获取到的是一个坏的连接,那么这个数据源允许这个线程尝试重新获取一个新的连接,但是这个重新尝试的次数不应该超过 poolMaximumIdleConnections 与 poolMaximumLocalBadConnectionTolerance 之和。 默认值:3 (新增于 3.4.5)
poolPingQuery – 发送到数据库的侦测查询,用来检验连接是否正常工作并准备接受请求。默认是“NO PING QUERY SET”,这会导致多数数据库驱动失败时带有一个恰当的错误消息。
poolPingEnabled – 是否启用侦测查询。若开启,需要设置 poolPingQuery 属性为一个可执行的 SQL 语句(最好是一个速度非常快的 SQL 语句),默认值:false。
poolPingConnectionsNotUsedFor – 配置 poolPingQuery 的频率。可以被设置为和数据库连接超时时间一样,来避免不必要的侦测,默认值:0(即所有连接每一时刻都被侦测 — 当然仅当 poolPingEnabled 为 true 时适用)。
PooledDataSource
PooledDataSource
完成池功能的类,直接看拿连接的popConnection
方法:
private PooledConnection popConnection(String username, String password) throws SQLException {
boolean countedWait = false;
PooledConnection conn = null;
// 触发获取连接的当前时间
long t = System.currentTimeMillis();
int localBadConnectionCount = 0;
while (conn == null) {
// 同步
synchronized (state) {
// 判断空闲列表中是否可以提供连接
if (!state.idleConnections.isEmpty()) {
// Pool has available connection
conn = state.idleConnections.remove(0);
if (log.isDebugEnabled()) {
log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");
}
} else {
// Pool does not have available connection
// 判断是否达到最大连接数限制
if (state.activeConnections.size() < poolMaximumActiveConnections) {
// Can create new connection
conn = new PooledConnection(dataSource.getConnection(), this);
if (log.isDebugEnabled()) {
log.debug("Created connection " + conn.getRealHashCode() + ".");
}
} else {
// Cannot create new connection
PooledConnection oldestActiveConnection = state.activeConnections.get(0);
long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
// 判断最老一个连接使用时间是否超过最大值
if (longestCheckoutTime > poolMaximumCheckoutTime) {
// Can claim overdue connection
state.claimedOverdueConnectionCount++;
state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;
state.accumulatedCheckoutTime += longestCheckoutTime;
state.activeConnections.remove(oldestActiveConnection);
if (!oldestActiveConnection.getRealConnection().getAutoCommit()) {
try {
oldestActiveConnection.getRealConnection().rollback();
} catch (SQLException e) {
/*
Just log a message for debug and continue to execute the following
statement like nothing happend.
Wrap the bad connection with a new PooledConnection, this will help
to not intterupt current executing thread and give current thread a
chance to join the next competion for another valid/good database
connection. At the end of this loop, bad {@link @conn} will be set as null.
*/
log.debug("Bad connection. Could not roll back");
}
}
// 这里看到将包装在oldestActiveConnection中的RealConnection重新用PooledConnection包装出来直接使用,看前面操作是将连接进行回滚,但是可能失败,却不关心,注释解释是,在后面的代码中会进行isValid的判断,其中就会判断连接是否可用。
conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this);
conn.setCreatedTimestamp(oldestActiveConnection.getCreatedTimestamp());
conn.setLastUsedTimestamp(oldestActiveConnection.getLastUsedTimestamp());
// 将老连接设置成invalid
oldestActiveConnection.invalidate();
if (log.isDebugEnabled()) {
log.debug("Claimed overdue connection " + conn.getRealHashCode() + ".");
}
} else {
// Must wait
try {
if (!countedWait) {
state.hadToWaitCount++;
countedWait = true;
}
if (log.isDebugEnabled()) {
log.debug("Waiting as long as " + poolTimeToWait + " milliseconds for connection.");
}
long wt = System.currentTimeMillis();
// 线程等待,也释放了锁
state.wait(poolTimeToWait);
state.accumulatedWaitTime += System.currentTimeMillis() - wt;
} catch (InterruptedException e) {
break;
}
}
}
}
if (conn != null) {
// ping to server and check the connection is valid or not
if (conn.isValid()) {
if (!conn.getRealConnection().getAutoCommit()) {
conn.getRealConnection().rollback();
}
conn.setConnectionTypeCode(assembleConnectionTypeCode(dataSource.getUrl(), username, password));
conn.setCheckoutTimestamp(System.currentTimeMillis());
conn.setLastUsedTimestamp(System.currentTimeMillis());
state.activeConnections.add(conn);
state.requestCount++;
state.accumulatedRequestTime += System.currentTimeMillis() - t;
} else {
if (log.isDebugEnabled()) {
log.debug("A bad connection (" + conn.getRealHashCode() + ") was returned from the pool, getting another connection.");
}
state.badConnectionCount++;
localBadConnectionCount++;
// 不可用的连接会被设置成null,被回收器回收
conn = null;
if (localBadConnectionCount > (poolMaximumIdleConnections + poolMaximumLocalBadConnectionTolerance)) {
if (log.isDebugEnabled()) {
log.debug("PooledDataSource: Could not get a good connection to the database.");
}
throw new SQLException("PooledDataSource: Could not get a good connection to the database.");
}
}
}
}
}
if (conn == null) {
if (log.isDebugEnabled()) {
log.debug("PooledDataSource: Unknown severe error condition. The connection pool returned a null connection.");
}
throw new SQLException("PooledDataSource: Unknown severe error condition. The connection pool returned a null connection.");
}
return conn;
}
popConnection
方法实现在一个池中获取连接的基本逻辑,依赖最大连接数,获取等待时间,连接使用超时时间等来完成一个池的核心能力。
注意这里使用wait
方法来等待,在java线程池中使用阻塞队列来出来暂时拿不到资源的请求。
前面提到,在使用Connection
时,调用close
方法,会调用到dataSource.pushConnection(this);
,就是将这个连接使用完了还回池的动作:
protected void pushConnection(PooledConnection conn) throws SQLException {
// 一样加锁
synchronized (state) {
// 从使用线程列表中删除
state.activeConnections.remove(conn);
if (conn.isValid()) {
// 判断空闲连接列表是否超过最大值
if (state.idleConnections.size() < poolMaximumIdleConnections && conn.getConnectionTypeCode() == expectedConnectionTypeCode) {
state.accumulatedCheckoutTime += conn.getCheckoutTime();
if (!conn.getRealConnection().getAutoCommit()) {
conn.getRealConnection().rollback();
}
PooledConnection newConn = new PooledConnection(conn.getRealConnection(), this);
// 加入到空闲连接列表中
state.idleConnections.add(newConn);
newConn.setCreatedTimestamp(conn.getCreatedTimestamp());
newConn.setLastUsedTimestamp(conn.getLastUsedTimestamp());
conn.invalidate();
if (log.isDebugEnabled()) {
log.debug("Returned connection " + newConn.getRealHashCode() + " to pool.");
}
// 通知等待线程
state.notifyAll();
} else {
state.accumulatedCheckoutTime += conn.getCheckoutTime();
if (!conn.getRealConnection().getAutoCommit()) {
conn.getRealConnection().rollback();
}
conn.getRealConnection().close();
if (log.isDebugEnabled()) {
log.debug("Closed connection " + conn.getRealHashCode() + ".");
}
conn.invalidate();
}
} else {
if (log.isDebugEnabled()) {
log.debug("A bad connection (" + conn.getRealHashCode() + ") attempted to return to the pool, discarding connection.");
}
state.badConnectionCount++;
}
}
}
归还连接时,需要查看空闲列表中的线程数量是否已经到到设置的最大值,如果已经达到,就不需要归还了,凡是需要加入空闲列表的都需要进行notifyAll
操作,来通知那些等待的线程来抢这个归还的连接,但是如果此时连接池中空闲连接充足,并没有线程等待,这个操作也就浪费了,所以可以思考前面popConnection
中的wait
和这里的notifyAll
是可以用等待队列来完成。
另外一个方法,用于判断连接是否可用:
protected boolean pingConnection(PooledConnection conn) {
boolean result = true;
try {
// 先用isClosed来获取结果
result = !conn.getRealConnection().isClosed();
} catch (SQLException e) {
if (log.isDebugEnabled()) {
log.debug("Connection " + conn.getRealHashCode() + " is BAD: " + e.getMessage());
}
result = false;
}
if (result) {
// 可以通过poolPingEnabled配置来决定是否使用自定义sql
if (poolPingEnabled) {
if (poolPingConnectionsNotUsedFor >= 0 && conn.getTimeElapsedSinceLastUse() > poolPingConnectionsNotUsedFor) {
try {
if (log.isDebugEnabled()) {
log.debug("Testing connection " + conn.getRealHashCode() + " ...");
}
Connection realConn = conn.getRealConnection();
Statement statement = realConn.createStatement();
// 执行poolPingQuery
ResultSet rs = statement.executeQuery(poolPingQuery);
rs.close();
statement.close();
if (!realConn.getAutoCommit()) {
realConn.rollback();
}
result = true;
if (log.isDebugEnabled()) {
log.debug("Connection " + conn.getRealHashCode() + " is GOOD!");
}
} catch (Exception e) {
log.warn("Execution of ping query '" + poolPingQuery + "' failed: " + e.getMessage());
try {
conn.getRealConnection().close();
} catch (Exception e2) {
//ignore
}
result = false;
if (log.isDebugEnabled()) {
log.debug("Connection " + conn.getRealHashCode() + " is BAD: " + e.getMessage());
}
}
}
}
}
return result;
}
从代码中可以看到isClosed
方法并不可靠,最终还是通过执行sql来判断连接是否可用,这个在很多涉及判断数据库连接是否有效的地方都是这么做的,详细可以看一下isClosed
方法的注释。
PooledDataSourceFactory
继承UnpooledDataSourceFactory,直接返回PooledDataSource对象
public class PooledDataSourceFactory extends UnpooledDataSourceFactory {
public PooledDataSourceFactory() {
this.dataSource = new PooledDataSource();
}
}
心得
在整个线程池的实现代码中,可以学习到一个池的实现的要素有哪些,以及录用基础代码如何实现一个池。对于那些封装成高层次的池的代码来说,这个实现显得又些单薄和不够全面,可是无论连接池如何实现核心池的实现逻辑是不会变的。