mybatis 核心组件介绍
- SqlSessionFactoryBuilder(构造器)：它可以通过 xml，注解或者手动配置来创建 SqlSessionFacotry
- SqlSessionFactory：用来创建 SqlSession(会话)的工厂
- SqlSession：SqlSession 是 mybatis 最核心的类，可以用来执行语句，提交或者回滚事务以及获取映射器 Mapper 的接口
- SQL Mapper：它是由一个接口，一个 xml 配置文件或者注解构成，需要给出对应的 SQL 和映射规则，它负责发送 SQL 去执行，并发挥结果
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组件使用案例：
public class MybatisTest { private static SqlSessionFactory sqlSessionFactory; static { try {sqlSessionFactory = new SqlSessionFactoryBuilder() .build(Resources.getResourceAsStream("mybatis-config.xml")); } catch (IOException e) {e.printStackTrace(); } } public static void main(String[] args) {try (SqlSession sqlSession = sqlSessionFactory.openSession()) {UserMapper userMapper = sqlSession.getMapper(UserMapper.class); User user = userMapper.selectById(1); System.out.println("User :" + user); } } } // 结果：User : User{id=1, age=21, name='pjmike'}
mybatis 动态代理实现
public static void main(String[] args) {try (SqlSession sqlSession = sqlSessionFactory.openSession()) {UserMapper userMapper = sqlSession.getMapper(UserMapper.class);// <1>
User user = userMapper.selectById(1);
System.out.println("User :" + user);
}
} 在前面的例子中，我们使用 sqlSession 的 getMapper 方法获取了 UserMapper 对象，实际上我们获取的是 UserMapper 接口的代理类，然后由代理类来执行方法。在探索动态代理类实现之前，我们需要先明确 sqlSessionFactory 工厂的创建过程做了哪些准备工作。
mybatis 全局配置文件解析
private static SqlSessionFactory sqlSessionFactory;
static {
try {sqlSessionFactory = new SqlSessionFactoryBuilder()
.build(Resources.getResourceAsStream("mybatis-config.xml"));
} catch (IOException e) {e.printStackTrace();
}
} 我们使用 new SqlSessionFactoryBuilder().build() 方法创建 SqlSessionFactory 工厂
public SqlSessionFactory build(InputStream inputStream, Properties properties) {return build(inputStream, null, properties);
}
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
return build(parser.parse());
} catch (Exception e) {throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {ErrorContext.instance().reset();
try {inputStream.close();
} catch (IOException e) {// Intentionally ignore. Prefer previous error.}
}
} 对于 mybatis 的全局配置文件解析，相关的解析代码主要在 XMLConfigBuilder 的parse()方法中
public Configuration parse() {if (this.parsed) {throw new BuilderException("Each XMLConfigBuilder can only be used once.");
} else {
this.parsed = true;
// 解析全局配置文件
this.parseConfiguration(this.parser.evalNode("/configuration"));
return this.configuration;
}
}
private void parseConfiguration(XNode root) {
try {this.propertiesElement(root.evalNode("properties"));
Properties settings = this.settingsAsProperties(root.evalNode("settings"));
this.loadCustomVfs(settings);
this.typeAliasesElement(root.evalNode("typeAliases"));
this.pluginElement(root.evalNode("plugins"));
this.objectFactoryElement(root.evalNode("objectFactory"));
this.objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
this.reflectorFactoryElement(root.evalNode("reflectorFactory"));
this.settingsElement(settings);
this.environmentsElement(root.evalNode("environments"));
this.databaseIdProviderElement(root.evalNode("databaseIdProvider"));
this.typeHandlerElement(root.evalNode("typeHandlers"));
// 解析映射器配置文件
this.mapperElement(root.evalNode("mappers"));
} catch (Exception var3) {throw new BuilderException("Error parsing SQL Mapper Configuration. Cause:" + var3, var3);
}
} 从 parseConfiguration 方法的源码中可以看出，XmlConfigBuilder读取 mybatis-config.xml 中的配置信息，然后将信息保存到 configuration 类中
映射器 Mapper 文件解析
// 解析映射器配置文件
this.mapperElement(root.evalNode("mappers")); 该方法是对全局配置文件中 mappers 属性的解析
private void mapperElement(XNode parent) throws Exception {if (parent != null) {Iterator var2 = parent.getChildren().iterator();
while(true) {while(var2.hasNext()) {XNode child = (XNode)var2.next();
String resource;
if ("package".equals(child.getName())) {resource = child.getStringAttribute("name");
this.configuration.addMappers(resource);
} else {resource = child.getStringAttribute("resource");
String url = child.getStringAttribute("url");
String mapperClass = child.getStringAttribute("class");
XMLMapperBuilder mapperParser;
InputStream inputStream;
if (resource != null && url == null && mapperClass == null) {ErrorContext.instance().resource(resource);
inputStream = Resources.getResourceAsStream(resource);
mapperParser = new XMLMapperBuilder(inputStream, this.configuration, resource, this.configuration.getSqlFragments());
mapperParser.parse();} else if (resource == null && url != null && mapperClass == null) {ErrorContext.instance().resource(url);
inputStream = Resources.getUrlAsStream(url);
mapperParser = new XMLMapperBuilder(inputStream, this.configuration, url, this.configuration.getSqlFragments());
mapperParser.parse();} else {if (resource != null || url != null || mapperClass == null) {throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
}
Class<?> mapperInterface = Resources.classForName(mapperClass);
this.configuration.addMapper(mapperInterface);
}
}
}
return;
}
}
} 其中的 mapperParser.parse() 方法就是 XmlMapperBuilder 对映射器文件的解析
public void parse() {if (!this.configuration.isResourceLoaded(this.resource)) {
// 解析映射文件的 mapper 节点，该方法主要用于将 mapper 文件中的元素信息解析到 MappedStatement 对象，并保存到 configuration 类的 mappedStatements 属性中
this.configurationElement(this.parser.evalNode("/mapper"));
this.configuration.addLoadedResource(this.resource);
// 重点方法，这个方法内部会根据 namespace 属性值，生成动态代理类
this.bindMapperForNamespace();}
this.parsePendingResultMaps();
this.parsePendingCacheRefs();
this.parsePendingStatements();} 核心方法：bindMapperForNamespace()方法，该方法会根据 mapper 文件中的 namespace 属性值，为接口类生成动态代理类
动态代理类的生成
private void bindMapperForNamespace() {
// 获取 mapper 元素的 namespace 的元素值
String namespace = this.builderAssistant.getCurrentNamespace();
if (namespace != null) {
Class boundType = null;
try {boundType = Resources.classForName(namespace);
} catch (ClassNotFoundException var4) {// 如果没有这个类，可以直接忽略，这是因为 namespace 属性值只需要保持唯一就可以了，并不一定对应一个 XXXMapper 接口，没有 XXXMapper 接口时，我们可以直接使用 SqlSession 来进行增删改查}
if (boundType != null && !this.configuration.hasMapper(boundType)) {this.configuration.addLoadedResource("namespace:" + namespace);
// 如果 namespace 属性值有对应的 java 类，调用 configuration 中的 addMapper 方法，将其添加到 MapperRegistry 中
this.configuration.addMapper(boundType);
}
}
}public <T> void addMapper(Class<T> type) {
// 这个类必须是一个 class 接口，因为使用的是 JDK 动态代理，所以需要接口，否则不会针对其生成动态代理
if (type.isInterface()) {if (this.hasMapper(type)) {throw new BindingException("Type" + type + "is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
// 用于生成一个 MapperProxyFacotry, 用于后面生成动态代理类
this.knownMappers.put(type, new MapperProxyFactory(type));
// 以下代码块主要用于解析我们定义的 XXXMapper 接口里面使用的注解，这里主要处理不使用 xml 映射文件的情况
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(this.config, type);
parser.parse();
loadCompleted = true;
} finally {if (!loadCompleted) {this.knownMappers.remove(type);
}
}
}
} MapperRegistry内部维护了一个映射关系，每个接口对应一个 MapperProxyFactory(生成动态代理工厂类)
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap(); 这样便于在后面调用 MapperRegistry 的getMapper()时，直接从 Map 中获取某一个接口对应的动态代理工厂类，然后再利用工厂类对其接口生成真正的动态代理类
Configuration 的 getMapper()方法
开始我们通过 sqlsession 的 getMapper() 方法调用获取到动态代理类
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
public class DefaultSqlSession implements SqlSession {
private final Configuration configuration;
private final Executor executor;
@Override
public <T> T getMapper(Class<T> type) {return configuration.getMapper(type, this);
}
...
} Configuration中的 getMapper() 方法内部其实是使用的 MapperRegistry 的getMapper()方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
if (mapperProxyFactory == null) {throw new BindingException("Type" + type + "is not known to the MapperRegistry.");
} else {
try {
// 这里可以看到，每次调用都会生成一个新的代理对象返回
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception var5) {throw new BindingException("Error getting mapper instance. Cause:" + var5, var5);
}
}
}protected T newInstance(MapperProxy<T> mapperProxy) {
// 这里使用 JDK 动态代理，通过 Proxy.newProxyInstance 生成动态代理类
//newProxyInstance 的参数：类加载器，接口类，InvocationHandler 接口实现类
// 动态代理可以将所有的接口的调用重定向到调用处理器 InvocationHandler，调用它的 invoke 方法
return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
return this.newInstance(mapperProxy);
} 这里就是 InvocationHandler 接口的实现类 MapperProxy
public class MapperProxy<T> implements InvocationHandler, Serializable {
private static final long serialVersionUID = -6424540398559729838L;
private final SqlSession sqlSession;
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache;
public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
// 如果调用的是 Object 类中定义的方法，直接通过反射调用即可
if (Object.class.equals(method.getDeclaringClass())) {return method.invoke(this, args);
}
if (this.isDefaultMethod(method)) {return this.invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable var5) {throw ExceptionUtil.unwrapThrowable(var5);
}
// 调用 XXXMapper 接口自定义的方法，进行代理
// 首先将当前被调用的方法构造成一个 MapperMethod 对象，然后调用其 excute 方法真正开始执行
MapperMethod mapperMethod = this.cachedMapperMethod(method);
return mapperMethod.execute(this.sqlSession, args);
}
...
}public Object execute(SqlSession sqlSession, Object[] args) {
Object param;
Object result;
switch(this.command.getType()) {
case INSERT:
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));
break;
case UPDATE:
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.update(this.command.getName(), param));
break;
case DELETE:
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));
break;
case SELECT:
if (this.method.returnsVoid() && this.method.hasResultHandler()) {this.executeWithResultHandler(sqlSession, args);
result = null;
} else if (this.method.returnsMany()) {result = this.executeForMany(sqlSession, args);
} else if (this.method.returnsMap()) {result = this.executeForMap(sqlSession, args);
} else if (this.method.returnsCursor()) {result = this.executeForCursor(sqlSession, args);
} else {param = this.method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(this.command.getName(), param);
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for:" + this.command.getName());
}
if (result == null && this.method.getReturnType().isPrimitive() && !this.method.returnsVoid()) {throw new BindingException("Mapper method'" + this.command.getName() + "attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ").");
} else {return result;}
} 在 MapperMethod 中还有两个内部类，SqlCommand 和 MethodSignature 类，在 excute 方法中首先使用 swithc case 语句根据 SqlCommand 的getType()方法，判断要执行的 sql 类型，然后调用 SqlSession 的增删改查方法
getMapper()方法流程图
参考资料 & 鸣谢
- https://pjmike.github.io/2018…