首先,当我们调用getMapper的时候,就会进入invoke()方法:
// 5.操作Mapper接口
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable var5) {
throw ExceptionUtil.unwrapThrowable(var5);
}
} else {
MapperMethod mapperMethod = this.cachedMapperMethod(method); //将我们的代理方法缓存起来
return mapperMethod.execute(this.sqlSession, args);
}
}
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = (MapperMethod)this.methodCache.get(method);
if (mapperMethod == null) {
mapperMethod = new MapperMethod(this.mapperInterface, method, this.sqlSession.getConfiguration());
this.methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
缓存的目的:知道SQL语句对应的mapper接口中的方法
下面看看execute()方法
public Object execute(SqlSession sqlSession, Object[] args) {
....
} else {
param = this.method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(this.command.getName(), param); //最终调用selectOne()方法
}
.....
}
到了本篇文章的重点了,下面我们就开始分析selectOne()方法里面怎么实现的。
通过源码分析我们可以知道mapper.getUser()就是调用selectOne()方法。所以下面的两行代码是等效的,效果一样
UserEntity user = mapper.getUser(2);
UserEntity o = sqlSession.selectOne("com.mayikt.mapper.UserMapper.getUser", 2);
进入selectOne()可知,SqlSession这个接口帮我们封装了CRUD的方法,便于我们操作。
最终执行DefaultSqlSession,因为前面new了DefaultSqlSessionFactory()
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
下面会执行这段代码
public SqlSession openSession() {
return this.openSessionFromDataSource(this.configuration.getDefaultExecutorType(), (TransactionIsolationLevel)null, false);//进入这里
}
所以,通过上述分析,我们知道这里就是执行DefaultSqlSession
public <T> T selectOne(String statement, Object parameter) {
List<T> list = this.selectList(statement, parameter);//进入这里
if (list.size() == 1) {
return list.get(0);
} else if (list.size() > 1) {
throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
} else {
return null;
}
}
底层还是查询所有的,但是还是取第一个,查询多个的化就会抛出异常。
public <E> List<E> selectList(String statement, Object parameter) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
List var5;
try {
MappedStatement ms = this.configuration.getMappedStatement(statement);//这里得到sql语句
var5 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception var9) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + var9, var9);
} finally {
ErrorContext.instance().reset();
}
return var5;
}
通过上面的RowBounds,我们可知这个是分页用到的类。得出结论:Mybatis默认最多能查到Integer的最大值
下面进入这个方法
MappedStatement ms = this.configuration.getMappedStatement(statement);
MappedStatement存放的 SQL语句的配置内容,到query()方法的时候,有两个实现类的方法,我们应该走哪一个方法,下面我们开始debug源码分析这块
下面是debug到getMappedStatement()这块的MappedStatement
由此我们可知:MappedStatement是存放我们SQL的配置内容
我们可以知道了,executor为CachingExecutor,我们再来看看Executor接口下面有哪些Executor执行器
这里先透露下:CachingExecutor为二级缓存执行器,BaseExecutor为一级缓存执行器。
下面我们来分析下原因:先回到我们的openSession()方法
// 4.获取Session
SqlSession sqlSession = sqlSessionFactory.openSession();
public SqlSession openSession() {
return this.openSessionFromDataSource(this.configuration.getDefaultExecutorType(), (TransactionIsolationLevel)null, false);
}
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
DefaultSqlSession var8;
try {
Environment environment = this.configuration.getEnvironment();
TransactionFactory transactionFactory = this.getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
Executor executor = this.configuration.newExecutor(tx, execType); //看到没有,看到没用
var8 = new DefaultSqlSession(this.configuration, executor, autoCommit);
} catch (Exception var12) {
this.closeTransaction(tx);
throw ExceptionFactory.wrapException("Error opening session. Cause: " + var12, var12);
} finally {
ErrorContext.instance().reset();
}
return var8;
}
Executor executor = this.configuration.newExecutor(tx, execType);
上述代码,在我们创建sqlSession的时候帮我们创建好了执行器,进入这个方法:
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? this.defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Object executor;
if (ExecutorType.BATCH == executorType) { //批处理执行器
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction); //简单执行器
}
if (this.cacheEnabled) {
executor = new CachingExecutor((Executor)executor); //开启了缓存,则开启缓存执行器
}
Executor executor = (Executor)this.interceptorChain.pluginAll(executor);
return executor;
}
由此我们知道开启了缓存执行器,executor传递的是简单执行器,我们就明白了,先有简单执行器(SimpleExecutor),判断是否开启了二级缓存,开启了就创建缓存执行器(CacheExecutor)
最后返回executor
下面我们总结下这几个执行器的作用:
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = this.createCacheKey(ms, parameterObject, rowBounds, boundSql);
return this.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
进入这行代码:
CacheKey key = this.createCacheKey(ms, parameterObject, rowBounds, boundSql);
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
return this.delegate.createCacheKey(ms, parameterObject, rowBounds, boundSql);
}
由此我们知道,这里使用了多态的思想,没有SimpleExecutor执行器,说明走的是SimpleExecutor的父类BaseExecutor执行器的方法
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
if (this.closed) {
throw new ExecutorException("Executor was closed.");
} else {
CacheKey cacheKey = new CacheKey();
cacheKey.update(ms.getId());
cacheKey.update(rowBounds.getOffset());
cacheKey.update(rowBounds.getLimit());
cacheKey.update(boundSql.getSql());
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
for(int i = 0; i < parameterMappings.size(); ++i) {
ParameterMapping parameterMapping = (ParameterMapping)parameterMappings.get(i);
if (parameterMapping.getMode() != ParameterMode.OUT) {
String propertyName = parameterMapping.getProperty();
Object value;
if (boundSql.hasAdditionalParameter(propertyName)) {
value = boundSql.getAdditionalParameter(propertyName);
} else if (parameterObject == null) {
value = null;
} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
value = parameterObject;
} else {
MetaObject metaObject = this.configuration.newMetaObject(parameterObject);
value = metaObject.getValue(propertyName);
}
cacheKey.update(value);
}
}
if (this.configuration.getEnvironment() != null) {
cacheKey.update(this.configuration.getEnvironment().getId());
}
return cacheKey; //返回了cacheKey为:-978696406:1452564227:com.mayikt.mapper.UserMapper.getUser:0:2147483647:select * from user where id=?:2:development
}
}
通过上面得到了缓存key,得到key后,再调用query方法去查询:
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = this.createCacheKey(ms, parameterObject, rowBounds, boundSql);
return this.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) {
this.flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
this.ensureNoOutParams(ms, parameterObject, boundSql);
List<E> list = (List)this.tcm.getObject(cache, key);
if (list == null) {
list = this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
this.tcm.putObject(cache, key, list);
}
return list;
}
}
return this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
通过上述代码:我们可知,如果二级缓存没有,走简单执行器
二级缓存没用,则进入else分支:
return this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
思考:为什么CachingExecutor要找SimpleExecutor创建缓存key?
答案是为了复用,实现缓存key代码复用。mybatis缓存控制:先查找二级缓存(需要自己配置),二级缓存没有的情况下,再去找一级缓存(默认都有)
一级缓存是绝对有的,二级缓存(硬盘、Redis、EHCache)是可以没有的(表示没用使用,配置存储介质,就不会缓存,相当于空壳的)。
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 (this.closed) {
throw new ExecutorException("Executor was closed.");
} else {
if (this.queryStack == 0 && ms.isFlushCacheRequired()) {
this.clearLocalCache();
}
List list;
try {
++this.queryStack; //记录次数,保证安全
list = resultHandler == null ? (List)this.localCache.getObject(key) : null; //先去缓存中查,这个缓存指的是一级缓存
if (list != null) {
this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
--this.queryStack;
}
if (this.queryStack == 0) {
Iterator i$ = this.deferredLoads.iterator();
while(i$.hasNext()) {
BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)i$.next();
deferredLoad.load();
}
this.deferredLoads.clear();
if (this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
this.clearLocalCache();
}
}
return list;
}
}
执行从HashMap中查找缓存
list = resultHandler == null ? (List)this.localCache.getObject(key) : null;
public Object getObject(Object key) {
return this.cache.get(key);
}
所以,我们知道了PerpetualCache指的是我们的一级缓存,一级缓存指的是本地缓存,存放在内存中的。使用Map集合存放的。
我们知道,我们一级缓存现在也没有,所以会先往数据库中查询一次
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
this.localCache.putObject(key, ExecutionPlaceholder.EXECUTION_PLACEHOLDER); //这里缓存中先放个占位符,表示要去查询数据库了
List list;
try {
list = this.doQuery(ms, parameter, rowBounds, resultHandler, boundSql); //这里去数据库查询结果
} finally {
this.localCache.removeObject(key); //先删除占位key
}
this.localCache.putObject(key, list); //再存到缓存中
if (ms.getStatementType() == StatementType.CALLABLE) {
this.localOutputParameterCache.putObject(key, parameter);
}
return list;
}
最后回到前面:
protected int queryStack = 0;
这里queryStack为全局的,存在线程安全问题。
下面我们开启日志,来验证下本地一级缓存作用:在Mybatis配置文件加入下面配置,开启打印日志:
<settings>
<setting name="logImpl" value="STDOUT_LOGGING"/>
</settings>
结果:说明只发出一条SQL语句去数据库查询一次,第一次去查询数据库,将查询结果集存放在缓存中,第二次查询就直接走本地缓存查询。
第一次调用.... Opening JDBC Connection Created connection 1076835071. Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@402f32ff] ==> Preparing: select * from user where id=? ==> Parameters: 1(Integer) <== Columns: id, name, update_time <== Row: 1, xuyu, 2019-03-13 14:27:49.0 <== Total: 1 xuyu 第二次调用.... xuyu
加入在中间加入一条update语句,结果是怎样?
//中间执行一条update语句
sqlSession.update("com.mayikt.mapper.UserMapper.updateUser",1);
结果:发出了三条SQL语句。
第一次调用.... Opening JDBC Connection Created connection 1076835071. Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@402f32ff] ==> Preparing: select * from user where id=? ==> Parameters: 1(Integer) <== Columns: id, name, update_time <== Row: 1, xuyu, 2019-03-13 14:27:49.0 <== Total: 1 xuyu ==> Preparing: update user set name ='xiaoxu' where id=? ==> Parameters: 1(Integer) <== Updates: 1 第二次调用.... ==> Preparing: select * from user where id=? ==> Parameters: 1(Integer) <== Columns: id, name, update_time <== Row: 1, xiaoxu, 2019-03-13 14:27:49.0 <== Total: 1 xiaoxu
为什么是三条SQL语句?不是有缓存吗?
我们的注意:sqlSession缓存为了防止脏数据,在我们进行增加、修改、删除的时候,都会清除一级缓存。下面我们看下源码。
public int update(String statement, Object parameter) {
int var4;
try {
this.dirty = true;
MappedStatement ms = this.configuration.getMappedStatement(statement);
var4 = this.executor.update(ms, this.wrapCollection(parameter));
} catch (Exception var8) {
throw ExceptionFactory.wrapException("Error updating database. Cause: " + var8, var8);
} finally {
ErrorContext.instance().reset();
}
return var4;
}
var4 = this.executor.update(ms, this.wrapCollection(parameter));
public int update(MappedStatement ms, Object parameter) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
if (this.closed) {
throw new ExecutorException("Executor was closed.");
} else {
this.clearLocalCache(); //清除所有一级缓存
return this.doUpdate(ms, parameter);
}
}
public void clearLocalCache() {
if (!this.closed) {
this.localCache.clear();
this.localOutputParameterCache.clear();
}
}
protected PerpetualCache localCache;
protected PerpetualCache localOutputParameterCache;
我们就明白了,PerpetualCache都会被清除掉了。
1、线程安全问题 2、集群会产生问题(主要的)
如上图,会存在脏读问题。
所有我们怎么解决呢?集群的情况下,我们可以不去使用一级缓存,是不是可以直接关闭一级缓存?
答案是:不可用直接关闭一级缓存,Mybatis默认走SimpleExecutor,不能直接关闭一级缓存。
那么如何去关闭一级缓存?
方案1 在sql语句上 随机生成 不同的参数 存在缺点:map集合可能爆 内存溢出的问题 方案2 开启二级缓存 方案3 使用sqlSession强制清除缓存 方案4 创建新的sqlSession连接。
方案1:案例演示:
<select id="getUser" parameterType="map"
resultType="com.mayikt.entity.UserEntity">
select * from user where id=#{id} and #{randomString}=#{randomString}
</select>
System.out.println("第一次调用....");
Map randomMap=new HashMap();
randomMap.put("randomString",new Random().nextInt());
randomMap.put("id",1);
UserEntity o = sqlSession.selectOne("com.mayikt.mapper.UserMapper.getUser", randomMap);
System.out.println(o.getName());
//中间执行一条update语句
//sqlSession.update("com.mayikt.mapper.UserMapper.updateUser",1);
System.out.println("第二次调用....");
randomMap.put("randomString",new Random().nextInt());
UserEntity o2 = sqlSession.selectOne("com.mayikt.mapper.UserMapper.getUser", randomMap);
System.out.println(o2.getName());
输出结果
第一次调用.... Opening JDBC Connection Created connection 1463757745. Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@573f2bb1] ==> Preparing: select * from user where id=? and ?=? ==> Parameters: 1(Integer), 1705142735(Integer), 1705142735(Integer) <== Columns: id, name, update_time <== Row: 1, xuyu, 2019-03-13 14:27:49.0 <== Total: 1 xuyu 第二次调用.... ==> Preparing: select * from user where id=? and ?=? ==> Parameters: 1(Integer), -13684383(Integer), -13684383(Integer) <== Columns: id, name, update_time <== Row: 1, xuyu, 2019-03-13 14:27:49.0 <== Total: 1 xuyu
1、调用getMapper方法时候,会执行invoke方法,将我们的代理方法缓存起来 2、调用execute方法,最终执行selectOne方法 3、进入selectOne方法可知,sqlSession这个接口帮我们封装了CRUD的方法,便于我们操作SQL语句。 4、selectOne方法底层还是执行selectList方法查询所有,但取第一个 5、进入selectList方法,通过configuration得到SQL语句,再执行query方法 6、进入query方法,先执行CacheExecutor二级缓存执行器,发现没用配置二级缓存介质,则走SimpleExecutor简单执行器(一级缓存) 7、从HashMap中查找数据,一级缓存也没用数据,则会去查询数据库,查询到了数据,缓存到一级缓存 8、此时再去查询,就直接查询一级缓存数据(本地缓存)不会去查询数据库