这边博客衔接上一篇mybatis的xml解析的博客，在xml解析完成之后，首先会解析成一个Configuration对象，然后创建一个DefaultSqlSessionFactory的session工厂。在这一切的准备过程完成之后，就可以开始对数据库的操作了。
首先看openSession()方法
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();
}
}
首先，根据configuration中取出的environment ，然后获取一个TransactionFactory，接着通过事务工厂新建一个事务对象，其实在这一个步骤，并没有对数据库进行操作newTransaction方法仅仅是返回了一个Transaction对象，这个对象包含了Datasource, level,autocommit这几个属性，并没有做其他操作。（这里我xml中配置了JDBC事务，具体看这个事务，而不是第三方的事务）。
接下来创建一个Executor对象
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
当前肯定是创建了一个默认的Executor，就是SimpleExecutor，然后往下，判断是否配置了开启缓存，是的话则通过装饰器模式创建一个CachingExecutor，接着调用interceptorChain.pluginAll方法返回一个被层层代理的对象，这部分在上一篇博客中分析过。返回executor对象，再接下来，new了一个DefaultSqlSession再返回，至此openSession方法就执行结束了。接下来我们就可以调用DefaultSqlSession的select或者update等方法操作数据库了，不过还是看比较主流的方法。
BlogMapper mapper = session.getMapper(BlogMapper.class);
Blog blog = mapper.selectBlog(101);
首先看getMapper方法，调用的是configuration对象中mapRegister的getMapper方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
所以在上一篇博客中看到哪个addMapper方法，存放的是一个MapperProxyFactory工厂，就是因为这里每次getMapper会从对应的工厂中创建代理，这里是Proxy动态代理
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
先返回，此时已经获取到了BlogMapper的代理对象，然后执行selectBlog方法，这时候会执行到之前的代理方法中，找到之前的
MapperProxy类的invoke方法
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else {
return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
这里肯定不是Object类，所以执行cachedInvoker()
private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {
try {
return methodCache.computeIfAbsent(method, m -> {
if (m.isDefault()) {
try {
if (privateLookupInMethod == null) {
return new DefaultMethodInvoker(getMethodHandleJava8(method));
} else {
return new DefaultMethodInvoker(getMethodHandleJava9(method));
}
} catch (IllegalAccessException | InstantiationException | InvocationTargetException
| NoSuchMethodException e) {
throw new RuntimeException(e);
}
} else {
return new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
}
});
} catch (RuntimeException re) {
Throwable cause = re.getCause();
throw cause == null ? re : cause;
}
}
computeIfAbsent，这是jdk8的语法，大概就是看map中有没有这个key，没有就新建一个并返回新建的这个，有就直接返回，所以这里就是对方法会做一个缓存。现在是第一次执行，肯定是没有，所以会执行后面的创建方法。m.isDefault这些是兼容jdk8以上的接口的默认方法，实现是直接运行那个默认方法。
直接看PlainMethodInvoker，进入new MapperMethod
public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
this.command = new SqlCommand(config, mapperInterface, method);
this.method = new MethodSignature(config, mapperInterface, method);
}
先看SqlCommand的创建
public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
final String methodName = method.getName();
final Class<?> declaringClass = method.getDeclaringClass();
MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass,
configuration);
if (ms == null) {
if (method.getAnnotation(Flush.class) != null) {
name = null;
type = SqlCommandType.FLUSH;
} else {
throw new BindingException("Invalid bound statement (not found): "
+ mapperInterface.getName() + "." + methodName);
}
} else {
name = ms.getId();
type = ms.getSqlCommandType();
if (type == SqlCommandType.UNKNOWN) {
throw new BindingException("Unknown execution method for: " + name);
}
}
}
首先，先从configuration中查找出对应的MappedStatement，查找的过程是这样的，先查看当前的类是否存在对应的MappedStatement，如果有直接返回，否则从父类中查找是否有对应的MappedStatement
private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName,
Class<?> declaringClass, Configuration configuration) {
String statementId = mapperInterface.getName() + "." + methodName;
if (configuration.hasStatement(statementId)) {
return configuration.getMappedStatement(statementId);
} else if (mapperInterface.equals(declaringClass)) {
return null;
}
for (Class<?> superInterface : mapperInterface.getInterfaces()) {
if (declaringClass.isAssignableFrom(superInterface)) {
MappedStatement ms = resolveMappedStatement(superInterface, methodName,
declaringClass, configuration);
if (ms != null) {
return ms;
}
}
}
return null;
}
}
SqlCommand创建完成之后，再看MethodSignature的创建
public MethodSignature(Configuration configuration, Class<?> mapperInterface, Method method) {
Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface);
if (resolvedReturnType instanceof Class<?>) {
this.returnType = (Class<?>) resolvedReturnType;
} else if (resolvedReturnType instanceof ParameterizedType) {
this.returnType = (Class<?>) ((ParameterizedType) resolvedReturnType).getRawType();
} else {
this.returnType = method.getReturnType();
}
//返回类型是否是void
this.returnsVoid = void.class.equals(this.returnType);
//返回类型是否是集合
this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray();
//返回类型是否是游标
this.returnsCursor = Cursor.class.equals(this.returnType);
//返回类型是否是Optional
this.returnsOptional = Optional.class.equals(this.returnType);
//如果有@Mapkey,返回mapKey
this.mapKey = getMapKey(method);
//是否是Map
this.returnsMap = this.mapKey != null;
//找到第几个参数是RowBounds
this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
//找到第几个参数是ResultHandler
this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
//参数解析器,解析@Param中的名称
this.paramNameResolver = new ParamNameResolver(configuration, method);
}
创建完成之后，返回，调用PlainMethodInvoker的invoke方法，方法中调用的是mapperMethod的execute
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
switch (command.getType()) {
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
if (method.returnsOptional()
&& (result == null || !method.getReturnType().equals(result.getClass()))) {
result = Optional.ofNullable(result);
}
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}
这里看select方法
result = sqlSession.selectOne(command.getName(), param);
@Override
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List<T> list = this.selectList(statement, parameter);
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;
}
}
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
在这里调用executor的query方法，因为这里是个装饰者对象，所以看CachingExecutor的query方法
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);
}
@Override
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, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
首先从MappedStatement中获取Cache，这个Cache在解析xml的时候就已经创建了，如果获取到的不是null，那么首先执行flushCacheIfRequired，这个是通过在解析xml的时候判断是不是select决定的，除了select语句都是true,执行清除缓存，接下来从缓存中获取，如果有缓存，直接返回，如果没有，就执行查询。
接着看委托类的实现，这个委托类的实现在BaseExecutor中
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();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
}
这里又是一个Cache,不过这个Cache是mybatis内置的Cache，这就是常说的一级缓存，而这个一级缓存的清除，从代码上看，首先是配置了LocalCacheScope是STATEMENT的时候，默认是Session，然后就是当执行了close方法的时候。
再接着往下看，如果没有命中缓存，就会继续执行查询方法
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;
}
@Override
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.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
doQuery方法是实现类中的方法，当前是SimpleExecutor，先看StatementHandler 的获取
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
return statementHandler;
}
public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
switch (ms.getStatementType()) {
case STATEMENT:
delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case PREPARED:
delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case CALLABLE:
delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
default:
throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
}
}
首先看RoutingStatementHandler，是通过不同的StatementType创建不同的Handler处理器，MappedStatement 新建默认是PREPARED,CALLABLE是存储过程，STATEMENT就不说了，所以正常情况下创建的都是PreparedStatementHandler，进入构造方法
public PreparedStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
}
protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
this.configuration = mappedStatement.getConfiguration();
this.executor = executor;
this.mappedStatement = mappedStatement;
this.rowBounds = rowBounds;
this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
this.objectFactory = configuration.getObjectFactory();
if (boundSql == null) { // issue #435, get the key before calculating the statement
generateKeys(parameterObject);
boundSql = mappedStatement.getBoundSql(parameterObject);
}
this.boundSql = boundSql;
this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
}
重点是最后两行，参数处理器和结果处理器的创建
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
return parameterHandler;
}
public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
ResultHandler resultHandler, BoundSql boundSql) {
ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
return resultSetHandler;
}
创建之后，两者都有一个操作，就是使用interceptorChain.pluginAll进行了包装代理，
返回到newStatementHandler，interceptorChain.pluginAll对RoutingStatementHandler同样做了一个包装代理,继续返回
再往下看prepareStatement方法
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection, transaction.getTimeout());
handler.parameterize(stmt);
return stmt;
}
首先获取一个连接，接着执行handler的prepare方法，方法中调用的是委托类也就是PrepareStatementHandler的prepare方法，其是由父类实现的
@Override
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
ErrorContext.instance().sql(boundSql.getSql());
Statement statement = null;
try {
statement = instantiateStatement(connection);
setStatementTimeout(statement, transactionTimeout);
setFetchSize(statement);
return statement;
} catch (SQLException e) {
closeStatement(statement);
throw e;
} catch (Exception e) {
closeStatement(statement);
throw new ExecutorException("Error preparing statement. Cause: " + e, e);
}
}
这就是创建了一个JDBC的statement,接下来返回继续执行parameterize方法
public void parameterize(Statement statement) throws SQLException {
parameterHandler.setParameters((PreparedStatement) statement);
}
可以看到调用的是之前创建的ParameterHandler的setParameters方法，把参数设置到statement中，这里需要注意的是，虽然ParameterHandler被plugins代理比RoutingStatementHandler晚，但是实际上ParameterHandler方法的调用是在后面，所以拦截的顺序也在后面。
再往下看，接着会调用query方法
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
return resultSetHandler.handleResultSets(ps);
}
这里就是JDBC的执行了，最后看ResultSetHandler执行的handleResultSets方法
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
final List<Object> multipleResults = new ArrayList<>();
int resultSetCount = 0;
ResultSetWrapper rsw = getFirstResultSet(stmt);
List<ResultMap> resultMaps = mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount);
while (rsw != null && resultMapCount > resultSetCount) {
ResultMap resultMap = resultMaps.get(resultSetCount);
handleResultSet(rsw, resultMap, multipleResults, null);
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
String[] resultSets = mappedStatement.getResultSets();
if (resultSets != null) {
while (rsw != null && resultSetCount < resultSets.length) {
ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
if (parentMapping != null) {
String nestedResultMapId = parentMapping.getNestedResultMapId();
ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
handleResultSet(rsw, resultMap, null, parentMapping);
}
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
}
return collapseSingleResultList(multipleResults);
}
这段代码大概就是获取出resultsmap，然后对结果进行解析，所以重点是handeResultSet方法
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
try {
if (parentMapping != null) {
handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
} else {
if (resultHandler == null) {
DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
multipleResults.add(defaultResultHandler.getResultList());
} else {
handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
}
}
} finally {
// issue #228 (close resultsets)
closeResultSet(rsw.getResultSet());
}
}
首先判断parentMapping 是否null，null就是最外层的resultMap, 然后判断是否有自定义的resultHandler，有的话用自定义的，没有就用默认的，解析完成之后返回，一路回到SimpleExecutor中，最后会执行closeStatement方法关闭连接。
返回结果，执行结束，mybatis的执行流程也就结束了。
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