MyBatis源码浅析
posted on 2014-12-15 09:50 Tim-Tom
url: http://www.cnblogs.com/timlearn/p/4161567.html
什么是MyBatis
MyBatis是支持定制化SQL、存储过程以及高级映射的优秀的持久层框架。MyBatis 避免了几乎所有的 JDBC 代码和手工设置参数以及抽取结果集。MyBatis 使用简单的 XML 或注解来配置和映射基本体,将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。
MyBatis简单示例
虽然在使用MyBatis时一般都会使用XML文件,但是本文为了分析程序的简单性,简单的测试程序将不包含XML配置,该测试程序包含一个接口、一个启动类:
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public
interface
UserMapper {
@Select
(
"SELECT * FROM user WHERE id = #{id}"
)
User selectUser(
int
id);
}
public
class
Test2 {
public
static
void
main(String[] args) {
SqlSessionFactory sqlSessionFactory = initSqlSessionFactory();
SqlSession session = sqlSessionFactory.openSession();
try
{
User user = (User) session.selectOne(
"org.mybatis.example.UserMapper.selectUser"
,
1
);
System.out.println(user.getUserAddress());
System.out.println(user.getUserName());
}
finally
{
session.close();
}
}
private
static
SqlSessionFactory initSqlSessionFactory() {
DataSource dataSource =
new
PooledDataSource(
"com.mysql.jdbc.Driver"
,
"jdbc:mysql://127.0.0.1:3306/jdbc"
,
"root"
,
""
);
TransactionFactory transactionFactory =
new
JdbcTransactionFactory();
Environment environment =
new
Environment(
"development"
,
transactionFactory, dataSource);
Configuration configuration =
new
Configuration(environment);
configuration.addMapper(UserMapper.
class
);
SqlSessionFactory sqlSessionFactory =
new
SqlSessionFactoryBuilder()
.build(configuration);
return
sqlSessionFactory;
}
}
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UserMapper是一个接口,我们在构建sqlSessionFactory时通过configuration.addMapper(UserMapper.class)把该接口注册进了sqlSessionFactory中。从上面的代码中我们可以看出,要使用MyBatis,我们应该经过以下步骤:1、创建sqlSessionFactory(一次性操作);2、用sqlSessionFactory对象构造sqlSession对象;3、调用sqlSession的相应方法;4、关闭sqlSession对象。
在main方法中,我们没有配置sql,也没有根据查询结果拼接对象,只需在调用sqlSession方法时传入一个命名空间以及方法参数参数即可,所有的操作都是面向对象的。在UserMapper接口中,我们定制了自己的sql,MyBatis把书写sql的权利给予了我们,方便我们进行sql优化及sql排错。
JDBC基础回顾
直接使用JDBC是很痛苦的,JDBC连接数据库包含以下几个基本步骤:1、注册驱动 ;2、建立连接(Connection);3、创建SQL语句(Statement);4、执行语句;5、处理执行结果(ResultSet);6、释放资源,示例代码如下:
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public
static
void
test()
throws
SQLException{
// 1.注册驱动
Class.forName(
"com.mysql.jdbc.Driver"
);
// 2.建立连接 url格式 - JDBC:子协议:子名称//主机名:端口/数据库名?属性名=属性值&…
Connection conn = DriverManager.getConnection(
"jdbc:mysql://localhost:3306/jdbc"
,
"root"
,
""
);
// 3.创建语句
Statement st = conn.createStatement();
// 4.执行语句
ResultSet rs = st.executeQuery(
"select * from user"
);
// 5.处理结果
while
(rs.next()) {<br> User user =
new
User(rs.getObject(
1
), rs.getObject(
2
));
}
// 6.释放资源
rs.close();
st.close();
conn.close();
}
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可以看到与直接使用JDBC相比,MyBatis为我们简化了很多工作:
1、把创建连接相关工作抽象成一个sqlSessionFactory对象,一次创建多次使用;
2、把sql语句从业务层剥离,代码逻辑更加清晰,增加可维护性;
3、自动完成结果集处理,不需要我们编写重复代码。
但是,我们应该知道的是,框架虽然能够帮助我们简化工作,但是框架底层的代码肯定还是最基础的JDBC代码,因为这是Java平台连接数据库的通用方法,今天我将分析一下MyBatis源码,看看MyBatis是如何把这些基础代码封装成一个框架的。
MyBatis调用流程
我们最终调用的是sqlSession对象上的方法,所以我们先跟踪sqlSession的创建方法:sqlSessionFactory.openSession(),最终这个方法会调用到DefaultSqlSessionFactory的以下方法:
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private
SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level,
boolean
autoCommit) {
Transaction tx =
null
;
try
{
final
Environment environment = configuration.getEnvironment();
final
TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
final
Executor executor = configuration.newExecutor(tx, execType);
return
new
DefaultSqlSession(configuration, executor, autoCommit);
}
catch
(Exception e) {
closeTransaction(tx);
// may have fetched a connection so lets call close()
throw
ExceptionFactory.wrapException(
"Error opening session. Cause: "
+ e, e);
}
finally
{
ErrorContext.instance().reset();
}
}
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最终返回的对象是一个DefaultSqlSession对象,在调试模式下,我们看到autoCommit为false,executor为CachingExecutor类型,在CachingExecutor里面有属性delegate,其类型为simpleExecutor:
现在,我们跟进DefaultSqlSession的selectOne()方法,查看该方法的调用流程,selectOne()方法又会调用selectList()方法:
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public
<E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try
{
MappedStatement ms = configuration.getMappedStatement(statement);
List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
return
result;
}
catch
(Exception e) {
throw
ExceptionFactory.wrapException(
"Error querying database. Cause: "
+ e, e);
}
finally
{
ErrorContext.instance().reset();
}
}
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可以看到要得到查询结果,最终还是要调用executor上的query方法,这里的executor是CachingExecutor实例,跟进程序得到如下代码:
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public
<E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler)
throws
SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return
query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
public
<E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws
SQLException {
Cache cache = ms.getCache();
if
(cache !=
null
) {
flushCacheIfRequired(ms);
if
(ms.isUseCache() && resultHandler ==
null
) {
ensureNoOutParams(ms, parameterObject, boundSql);
@SuppressWarnings
(
"unchecked"
)
List<E> list = (List<E>) tcm.getObject(cache, key);
if
(list ==
null
) {
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list);
// issue #578. Query must be not synchronized to prevent deadlocks
}
return
list;
}
}
return
delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
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MyBatis框架首先生成了一个boundSql和CacheKey,在boundSql中包含有我们传入的sql语句:
生成boundSql和CacheKey后会调用一个重载函数,在重载函数中,我们会检测是否有缓存,这个缓存是MyBatis的二级缓存,我们没有配置,那么直接调用最后一句delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql),前面说过这个delagate其实就是simpleExecutor,跟进去查看一下:
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public
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws
SQLException {
ErrorContext.instance().resource(ms.getResource()).activity(
"executing a query"
).object(ms.getId());
if
(closed)
throw
new
ExecutorException(
"Executor was closed."
);
if
(queryStack ==
0
&& ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try
{
queryStack++;
list = resultHandler ==
null
? (List<E>) localCache.getObject(key) :
null
;
if
(list !=
null
) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
}
else
{
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
}
finally
{
queryStack--;
}
if
(queryStack ==
0
) {
for
(DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
deferredLoads.clear();
// issue #601
if
(configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
clearLocalCache();
// issue #482
}
}
return
list;
}
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关键代码是以下三行:
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list = resultHandler ==
null
? (List<E>) localCache.getObject(key) :
null
;
if
(list !=
null
) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
}
else
{
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
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首先尝试从localCache中根据key得到List,这里的localCache是MyBatis的一级缓存,如果得不到则调用queryFromDatabase()从数据库中查询:
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private
<E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws
SQLException {
List<E> list;
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try
{
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
}
finally
{
localCache.removeObject(key);
}
localCache.putObject(key, list);
if
(ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return
list;
}
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其中关键代码是调用doQuery()代码,SimpleExecutor的doQuery()方法如下:
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public
<E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
throws
SQLException {
Statement stmt =
null
;
try
{
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog());
return
handler.<E>query(stmt, resultHandler);
}
finally
{
closeStatement(stmt);
}
}
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调用了prepareStatement方法,该方法如下:
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private
Statement prepareStatement(StatementHandler handler, Log statementLog)
throws
SQLException {
Statement stmt;
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection);
handler.parameterize(stmt);
return
stmt;
}
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终于,我们看到熟悉的代码了,首先得到Connection,然后从Connection中得到Statement,同时在调试模式下我们看到,我们的sql语句已经被设置到stmt中了:
现在Statement对象有了,sql也设置进去了,就只差执行以及对象映射了,继续跟进代码,我们会跟踪到org.apache.ibatis.executor.statement.
PreparedStatementHandler类的executor方法:
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public
<E> List<E> query(Statement statement, ResultHandler resultHandler)
throws
SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
return
resultSetHandler.<E> handleResultSets(ps);
}
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在这里,调用了ps.execute()方法执行sql,接下来调用的resultSetHandler.<E> handleResultSets(ps)方法明显是对结果集进行封装,我就不继续跟进了。
MyBatis的数据库连接池
上面一部分介绍了MyBatis执行的整体流程,这一部分打算讨论一个具体话题:MyBatis的数据库连接池。
我们知道,每次连接数据库时都创建Connection是十分耗费性能的,所以我们在写JDBC代码时,一般都会使用数据库连接池,把用过的Connection不是直接关闭,而是放入数据库连接池中,方便下次复用,开源的数据库连接池有DBCP、C3P0等,MyBatis也实现了自己的数据库连接池,在这一节我将探索一下MyBatis实现的数据库连接池源码。
跟进上一节的getConnection()方法,我们最终会进入JdbcTransaction的getConnection()方法,getConnection()方法又会调用openConnection()方法,而openConnection()又将调用dataSource的getConnection()方法:
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public
Connection getConnection()
throws
SQLException {
if
(connection ==
null
) {
openConnection();
}
return
connection;
}
protected
void
openConnection()
throws
SQLException {
if
(log.isDebugEnabled()) {
log.debug(
"Opening JDBC Connection"
);
}
connection = dataSource.getConnection();
if
(level !=
null
) {
connection.setTransactionIsolation(level.getLevel());
}
setDesiredAutoCommit(autoCommmit);
}
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这里的dataSource是PooledDataSource类型,跟进查看源码如下:
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public
Connection getConnection()
throws
SQLException {
return
popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection();
}
private
PooledConnection popConnection(String username, String password)
throws
SQLException {
//暂不分析
}
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可以看到,在这里我们返回的对象其实已经不是原生的Connection对象了,而是一个动态代理对象,是PooledConnection的一个属性,所有对对Connection对象的操作都将被PooledConnection拦截,我们可以查看PooledConnection的定义如下:
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class
PooledConnection
implements
InvocationHandler {
private
static
final
String CLOSE =
"close"
;
private
static
final
Class<?>[] IFACES =
new
Class<?>[] { Connection.
class
};
private
int
hashCode =
0
;
private
PooledDataSource dataSource;
private
Connection realConnection;
private
Connection proxyConnection;
private
long
checkoutTimestamp;
private
long
createdTimestamp;
private
long
lastUsedTimestamp;
private
int
connectionTypeCode;
private
boolean
valid;
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
);
}
public
void
invalidate() {
valid =
false
;
}
public
boolean
isValid() {
return
valid && realConnection !=
null
&& dataSource.pingConnection(
this
);
}
public
Connection getRealConnection() {
return
realConnection;
}
public
Connection getProxyConnection() {
return
proxyConnection;
}
public
int
getRealHashCode() {
if
(realConnection ==
null
) {
return
0
;
}
else
{
return
realConnection.hashCode();
}
}
public
int
getConnectionTypeCode() {
return
connectionTypeCode;
}
public
void
setConnectionTypeCode(
int
connectionTypeCode) {
this
.connectionTypeCode = connectionTypeCode;
}
public
long
getCreatedTimestamp() {
return
createdTimestamp;
}
public
void
setCreatedTimestamp(
long
createdTimestamp) {
this
.createdTimestamp = createdTimestamp;
}
public
long
getLastUsedTimestamp() {
return
lastUsedTimestamp;
}
public
void
setLastUsedTimestamp(
long
lastUsedTimestamp) {
this
.lastUsedTimestamp = lastUsedTimestamp;
}
public
long
getTimeElapsedSinceLastUse() {
return
System.currentTimeMillis() - lastUsedTimestamp;
}
public
long
getAge() {
return
System.currentTimeMillis() - createdTimestamp;
}
public
long
getCheckoutTimestamp() {
return
checkoutTimestamp;
}
public
void
setCheckoutTimestamp(
long
timestamp) {
this
.checkoutTimestamp = timestamp;
}
public
long
getCheckoutTime() {
return
System.currentTimeMillis() - checkoutTimestamp;
}
public
int
hashCode() {
return
hashCode;
}
public
boolean
equals(Object obj) {
if
(obj
instanceof
PooledConnection) {
return
realConnection.hashCode() == (((PooledConnection) obj).realConnection
.hashCode());
}
else
if
(obj
instanceof
Connection) {
return
hashCode == obj.hashCode();
}
else
{
return
false
;
}
}
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())) {
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."
);
}
}
}
|
可以看到这个类暴露了很多接口检测Connection状态,例如连接是否有效,连接创建时间最近使用连接等:
这个类实现了InvocationHandler接口,最主要的一个方法如下:
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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())) {
checkConnection();
}
return
method.invoke(realConnection, args);
}
catch
(Throwable t) {
throw
ExceptionUtil.unwrapThrowable(t);
}
}
}
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可以看到,PooledConnection会拦截close方法,当客户端调用close()方法时,程序不会关闭Connection,而是会调用dataSource.pushConnection(this)方法,该方法的实现如下:
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protected
void
pushConnection(PooledConnection conn)
throws
SQLException {
synchronized
(state) {
state.activeConnections.remove(conn);
if
(conn.isValid()) {
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
);
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."
);
}
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++;
}
}
}
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可以看到,首先会把Connection从活跃列表中删除,然后检测空闲列表的长度有没有达到最大长度(默认为5),若没有达到,把Connection放入空闲链表,否则关闭连接。这里的state是一个PoolState对象,该对象定义如下:
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public
class
PoolState {
protected
PooledDataSource dataSource;
protected
final
List<PooledConnection> idleConnections =
new
ArrayList<PooledConnection>();
protected
final
List<PooledConnection> activeConnections =
new
ArrayList<PooledConnection>();
protected
long
requestCount =
0
;
protected
long
accumulatedRequestTime =
0
;
protected
long
accumulatedCheckoutTime =
0
;
protected
long
claimedOverdueConnectionCount =
0
;
protected
long
accumulatedCheckoutTimeOfOverdueConnections =
0
;
protected
long
accumulatedWaitTime =
0
;
protected
long
hadToWaitCount =
0
;
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();
}
}
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可以看到最终我们的Connection对象是放在ArrayList中的,该类还提供一些接口返回连接池基本信息。
好了,现在我们可以回去看看PooledDataSource的popConnection方法了:
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private
PooledConnection popConnection(String username, String password)
throws
SQLException {
boolean
countedWait =
false
;
PooledConnection conn =
null
;
long
t = System.currentTimeMillis();
int
localBadConnectionCount =
0
;
while
(conn ==
null
) {
synchronized
(state) {
if
(state.idleConnections.size() >
0
) {
// 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
if
(state.activeConnections.size() < poolMaximumActiveConnections) {
// Can create new connection
conn =
new
PooledConnection(dataSource.getConnection(),
this
);
@SuppressWarnings
(
"unused"
)
//used in logging, if enabled
Connection realConn = conn.getRealConnection();
if
(log.isDebugEnabled()) {
log.debug(
"Created connection "
+ conn.getRealHashCode() +
"."
);
}
}
else
{
// Cannot create new connection
PooledConnection oldestActiveConnection = state.activeConnections.get(
0
);
long
longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
if
(longestCheckoutTime > poolMaximumCheckoutTime) {
// Can claim overdue connection
state.claimedOverdueConnectionCount++;
state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;
state.accumulatedCheckoutTime += longestCheckoutTime;
state.activeConnections.remove(oldestActiveConnection);
if
(!oldestActiveConnection.getRealConnection().getAutoCommit()) {
oldestActiveConnection.getRealConnection().rollback();
}
conn =
new
PooledConnection(oldestActiveConnection.getRealConnection(),
this
);
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();
state.wait(poolTimeToWait);
state.accumulatedWaitTime += System.currentTimeMillis() - wt;
}
catch
(InterruptedException e) {
break
;
}
}
}
}
if
(conn !=
null
) {
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++;
conn =
null
;
if
(localBadConnectionCount > (poolMaximumIdleConnections +
3
)) {
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;
}
|
可以看到获取Connection一共分以下几种情况:1、如果有空闲Connection,那么直接使用空闲Connection,否则2;2、如果活跃Connection没有达到活跃Connection的上限,那么创建一个新Connection并返回,否则3;3、如果达到活跃上限,且被检出的Connection检出时间过长,那么把该Connection置为失效,新创建一个Connection,否则4;4、等待空闲Connection。
至此,我们就把MyBatis的数据库连接池代码整理了一遍,其中有两个关键点:1、检出的Connection其实不是原生Connection,而是一个代理对象;2、存放Connection的容器是ArrayList,Connection的检出遵从先进先出原则。
MyBatis的缓存
这篇博客讲的很好,mark一下:http://www.cnblogs.com/fangjian0423/p/mybatis-cache.html
MyBatis的事务
首先回顾一下JDBC的事务知识。
JDBC可以操作Connection的setAutoCommit()方法,给它false参数,提示数据库启动事务,在下达一连串的SQL命令后,自行调用Connection的commit()方法,提示数据库确认(Commit)操作。如果中间发生错误,则调用rollback(),提示数据库撤销(ROLLBACK)所有执行。同时,如果仅想要撤回某个SQL执行点,则可以设置存储点(SAVEPOINT)。一个示范的事务流程如下:
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Connection conn = ...;
Savepoint point =
null
;
try
{
conn.setAutoCommit(
false
);
Statement stmt = conn.createStatement();
stmt.executeUpdate(
"INSERT INTO ..."
);
...
point = conn.setSavepoint();
stmt.executeUpdate(
"INSERT INTO ..."
);
...
conn.commit();
}
catch
(SQLException e) {
e.printStackTrace();
if
(conn !=
null
) {
try
{
if
(point ==
null
) {
conn.rollback();
}
else
{
conn.rollback(point);
conn.releaseSavepoint(point);
}
}
catch
(SQLException ex) {
ex.printStackTrace();
}
}
}
finally
{
...
if
(conn !=
null
) {
try
{
conn.setAutoCommit(
true
);
conn.close();
}
catch
(SQLException ex) {
ex.printStackTrace();
}
}
}
|
在MyBatis调用流程一节就写过,在调试模式下,我们看到autoCommit为false,所以每个sqlSession其实都是一个事务,这也是为什么每次做删、改、查时都必须调用commit的原因。