getMapper用法在熟悉不過了，指明一個DAO接口的class，調用他的方法，即可從數(shù)據(jù)庫中返回 ，是不是感覺很神奇？
其實都是通過JDK的動態(tài)代理來實現(xiàn)，getMapper返回的即一個代理對象，通常在寫動態(tài)代理時，代理對象處理完成后還有調用被代理對象的對應方法，而像Mybatis這種面向接口的思想，沒有被代理的對象，所以，Mybatis通過自己一系列操作后直接返回。
源碼分析
一、加載MappedStatement
首先Mybatis要加載mapper下的所有insert、select、update、delete標簽，并且封裝成一個個MappedStatement對象，保存在Configuration下的Map<String, MappedStatement>中，具體實現(xiàn)在 XMLStatementBuilder#parseStatementNode() 和 MapperBuilderAssistant#addMappedStatement() 的方法中(addMappedStatement參數(shù)不多，也就20個),其中 this.configuration.addMappedStatement()就是保存在Configuration配置類中
二、getMapper()
getMapper()從DefaultSqlSession下的getMapper()中開始，中間經過Configuration的getMapper()，最終調用到MapperRegistry下的getMapper();其中knownMappers中的值在上述addMapper()中被添加。
public class MapperRegistry {
private final Configuration config;
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap();
public MapperRegistry(Configuration config) {
this.config = config;
}
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//從已知的集合中返回mapper代理工廠，
MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
//為空則拋出異常
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
} else {
try {
//實例化代理對象
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception var5) {
throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
}
}
........
}
動態(tài)代理生成工廠，通過newInstance實例化代理對象
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap();
public MapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class<T> getMapperInterface() {
return this.mapperInterface;
}
public Map<Method, MapperMethod> getMethodCache() {
return this.methodCache;
}
protected T newInstance(MapperProxy<T> mapperProxy) {
return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
//JDK的動態(tài)代理實現(xiàn)，代理對象為MapperProxy。
MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
return this.newInstance(mapperProxy);
}
}
所以在調用DAO中方法時，都會轉入到MapperProxy的invoke方法下。
執(zhí)行過程中判斷了是不是java8中的default方法或者是不是Object下的方法。如果都不是，才去查詢，
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 {
//如果是Object中的方法，則直接執(zhí)行代理類的對象的對應方法
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
}
//如果是默認方法，也就是java8中的default方法
if (this.isDefaultMethod(method)) {
return this.invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable var5) {
throw ExceptionUtil.unwrapThrowable(var5);
}
//從緩存中獲取MapperMethod
MapperMethod mapperMethod = this.cachedMapperMethod(method);
return mapperMethod.execute(this.sqlSession, args);
}
這里先要從methodCache獲取對應DAO方法的MapperMethod。
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;
}
緊接著是生成MapperMethod.SqlCommand和MapperMethod.MethodSignature，兩個自己的內部類，
在SqlCommand中，更具id從Configuration中獲取MappedStatement，此時就是上述第一步中添加的MappedStatement，完成代碼層和XML中的關系映射，并把MappedStatement的id和type提取出來，如果沒有獲取到，拋出異常，也就是為什么DAO中有select，xml配置中沒有id為select拋出異常的原因。
public class MapperMethod {
private final MapperMethod.SqlCommand command;
private final MapperMethod.MethodSignature method;
public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
this.command = new MapperMethod.SqlCommand(config, mapperInterface, method);
this.method = new MapperMethod.MethodSignature(config, mapperInterface, method);
}
//.........
public static class SqlCommand {
private final String name;
private final SqlCommandType type;
public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
String methodName = method.getName();
Class<?> declaringClass = method.getDeclaringClass();
//從Configuration中獲取MappedStatement
MappedStatement ms = this.resolveMappedStatement(mapperInterface, methodName, declaringClass, configuration);
if (ms == null) {
if (method.getAnnotation(Flush.class) == null) {
throw new BindingException("Invalid bound statement (not found): " + mapperInterface.getName() + "." + methodName);
}
this.name = null;
this.type = SqlCommandType.FLUSH;
} else {
this.name = ms.getId();
this.type = ms.getSqlCommandType();
if (this.type == SqlCommandType.UNKNOWN) {
throw new BindingException("Unknown execution method for: " + this.name);
}
}
}
更具不同的標簽類型，執(zhí)行不同的方法
