Mybatis配置和接口映射原理

Mybatis可以把Mapper.xml文件直接映射到对应的接口，调用接口方法会自动去Mapper.xml文件中找到对应的标签，这个功能就是利用Java的动态代理在binding包中实现的。

一、注册Mapper

在初始化时会把获取到的Mapper接口注册到MapperRegistry，注册的时候创建一个Mapper代理工厂，这个工厂通过JDK的代理创建一个执行对象，创建代理需要的InvocationHandler为MapperProxy

//接口注册
public class MapperRegistry {
public <T> void addMapper(Class<T> type) {
//如果是接口
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
//放到map中, value为创建代理的工厂
knownMappers.put(type, new MapperProxyFactory<T>(type));
// It's important that the type is added before the parser is run
// otherwise the binding may automatically be attempted by the mapper parser. If the type is already known, it won't try.
//这里是解析Mapper接口里面的注解
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}
}

二、获取接口对象

从knownMappers中根据接口类型取出对应的代理创建工厂，用该工厂创建代理。

public class MapperRegistry {
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//取出MapperProxyFactory
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null)
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
try {
//创建代理
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
}

//创建代理的工厂
public class MapperProxyFactory<T> {
/**
* 需要创建代理的接口
*/
private final Class<T> mapperInterface;
/**
* 执行方法的缓存,不需要每次都创建MapperMethod
*/
private Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();

public MapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}

public Class<T> getMapperInterface() {
return mapperInterface;
}

public Map<Method, MapperMethod> getMethodCache() {
return methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
//创建代理, InvocationHanderl是MapperProxy
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface },
mapperProxy);
}
/**
* 传人sqlSession创建代理
* @param sqlSession
* @return
*/
public T newInstance(SqlSession sqlSession) {
//把代理执行需要用到的对象传入
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}

三、调用接口方法

调用代理方法会进入到MapperProxy的public Object invoke(Object proxy, Method method, Object[] args)方法

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 {
//如果方法是Object里面的则直接调用方法
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
//获取执行方法的封装对象
final MapperMethod mapperMethod = cachedMapperMethod(method);
//里面就是找到对应的sql 执行sql语句
return mapperMethod.execute(sqlSession, args);
}
//缓存, 不需要每次都创建
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
//传人配置参数
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
}

最终执行sql会进入到MapperMethod中execute方法：

//具体的根据接口找到配置文件标签的类
public class MapperMethod {

private final SqlCommand command;
private final MethodSignature method;

public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
//SqlCommand封装该接口方法需要执行sql的相关属性，如：id(name), 类型
this.command = new SqlCommand(config, mapperInterface, method);
//执行方法特性进行封装，用于构造sql参数，判断执行sql逻辑走哪条分支
this.method = new MethodSignature(config, method);
}

public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//先找到对应的执行sql类型, sqlSession会调用不同方法
if (SqlCommandType.INSERT == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
} else if (SqlCommandType.UPDATE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
} else if (SqlCommandType.DELETE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
} else if (SqlCommandType.SELECT == command.getType()) {//如果是查询, 需要对返回做判断处理
//根据方法的特性判断进入哪个执行分支
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 {
//只查一条数据
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
} else {
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;
}
}

上面就是根据接口、方法、配置参数找到对应的执行sql，并构造参数，解析执行结果，具体sql执行在sqlSession流程里面，后面再看。