在阅读这篇文章之前,建议先阅读一下我之前写的两篇文章,对理解这篇文章很有帮助,特别是Mybatis新手:
写给mybatis小白的入门指南
mybatis底层原理学习(一):SqlSessionFactory和SqlSession的创建过程
如果你想获得更好的阅读体验,可以点击这里:Mybatis底层原理学习(二):从源码角度分析一次查询操作过程
(1)在使用Mybatis操作数据库的时候,每一次的CRUD操作都会去获取一次映射配置文件(mapper xml文件)对应的sql映射。每一个sql映射在内存缓存中(创建SqlSessionFactory之前就缓存在内存中了)都会有唯一ID,就是sql映射所在xml文件的命名空间加上sql映射配置节点的id值。
(2)Mapper xml文件的命名空间使用的是类的全路径名,这样做的好处是可以全局唯一,又可以通过反射获取对应的Mapper类。可以理解成每一个mapper xml文件对应一个Mapper类。
(3)mapper xml文件每一个sql映射节点的id属性值对应类的一个方法。我们在配置sql映射的时候也必须这样做,因为Mybatis的底层就是使用反射机制来获取执行方法的全路径作为ID来获取sql的映射配置的。
(4)每一个和mapper xml文件关联的类,都是Mapper类,在执行过程,通过动态代理,执行对应的方法。Mybatis是如何判断哪些类是Mapper类的呢?其实只有在运行时才会知道。在加载Mybatis配置文件中,通过解析mapper xml文件缓存了所有的sql映射配置,在调用SqlSession的getMapper方法获取Mapper类的时候才会生成代理类。
现在,我们来从源码角度分析Mapper代理类的创建过程,demo源码在后面给出
demo示例:
public class Main {
private static final Logger LOGGER = LoggerFactory.getLogger(Main.class);
public static void main(String[] args) {
SqlSession sqlSession = MyBatisUtil.getSqlSession();
ArticleMapper mapper = sqlSession.getMapper(ArticleMapper.class);
Article article = mapper.selectOne(1);
LOGGER.info("title:" + article.getTitle() + " " + "content:" + article.getContent());
}
}
我们在这行代码处搭上断点:
ArticleMapper mapper = sqlSession.getMapper(ArticleMapper.class);
Debug进去,执行下面代码:
public T getMapper(Class type) {
return configuration.getMapper(type, this);
}
configuration持有Mybatis的基本配置信息,继续看看getMapper方法的执行:
public T getMapper(Class type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}
mapperRegistry缓存了所有的SQL映射配置信息,在加载解析Mybatis配置文件(例子是mybatis)和mapper xml文件的时候完成缓存的,继续看getMapper的执行:
public T getMapper(Class<T> type, SqlSession sqlSession) {
// 这里首先会获取Mapper代理类工厂,拿到代理工厂就创建代理类
final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
// 创建Mapper代理类
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
通过动态代理机制创建Mapper代理类
protected T newInstance(MapperProxy mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
到这里,动态代理类创建完成。
通过分析了源码执行过程,Mapper代理类的创建过程弄清楚了,大体就是通过从缓存中获取sql映射配置的id(类全路径名+方法名)来通过动态代理机制创建代理类,实际执行的CRUD是执行动态代理类的方法。
执行CRUD操作的时候,我们都会执行到动态代理类的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 if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
最后找到映射的方法,执行mapperMethod.execute(sqlSession, args)
。
通过代码我们可以看到,会根据执行方法的操作类型(CRUD)执行不同的逻辑处理。
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);
}
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;
}
我们分析一下查询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);
}
首先根据方法返回类型的不同执行不同的逻辑,最终会调用SqlSession的selectXXX方法,
public T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List 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;
}
}
List
这行代码逻辑处理:
public List selectList(String statement, Object parameter) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
继续进去:
public List 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();
}
}
到这一步,是调用执行器Executor的query方法:
public List 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);
}
进去query方法:
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 and #116
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
继续进去query方法:
public List 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 list;
try {
queryStack++;
list = resultHandler == null ? (List) 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;
}
真正访问数据库的是这行代码:list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
private List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List 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;
}
查询操作由doQuery方法处理,这段代码就接近原生JDBC操作了,首先会获取语句处理器,然后开始执行语句,执行完,还会对结果进行结果集处理,返回处理的结果集,这里就不多分析了
public List 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);
}
}
我们在使用Mybatis进行CRUD操作的时候,大体过程是这样:
- 解析基本配置文件和Sql映射配置文件(mapper xml)文件,缓存配置文件节点内容在内存(一般此步骤只会执行一次,多次调用都会复用缓存结果)
- 获取SqlSession,通过SqlSession来获取Mapper类,生成Mapper类的代理
- 执行CRUED操作
当然,这个过程Mybatis还做了很多事情,Sql的解析,结果集的处理……等操作我们在这篇文章不分析,后面会有文章分析。这篇文章目的是分析Mapper代理类的创建过程和简单分析一个查询操作的过程。
源码地址
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