Mapper机制从流程上分为3个步骤
1.启动时做初始化动作
2.运行时使用getMapper方法生成动态代理
3.调用sql执行过程
Mapper机制初始化
系统启动首先会初始化Configuration，我们从Configuration的初始化位置开始看。请先找到XMLConfigBuilder类。
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
这段方法，是xml解析过程。重点关注parseConfiguration。
private void parseConfiguration(XNode root) {
try {
// 此处省略部分代码
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
继续跟进mapperElement
private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
// 此处省略部分代码
configuration.addMapper(mapperInterface);
} else {
throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
}
}
}
}
}
继续跟进configuration.addMapper(mapperInterface);一路可以跟进到MapperRegistry类
public void addMappers(String packageName) {
addMappers(packageName, Object.class);
}
这个方法是要加载某个包下面的所有xxxMapper.java（也就是我们所写的比如UserMapper.java,这种DAO行为接口），为后续运行时生成动态代理类做准备。（我们在业务代码中常写的sqlSession.getMapper方法，就是在生成动态代理）
public class MapperRegistry {
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();
public <T> void addMapper(Class<T> type) {
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
// knownMappers<Mapper接口类型, Mapper代理的工场对象>
// 初始化时，会把这些东西缓存起来。为后续生成动态代理做准备。
knownMappers.put(type, new MapperProxyFactory<T>(type));
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}
}
关注下knownMappers，这里面的key和value分别是<Mapper接口类型, Mapper代理的工场对象>
初始化时，会把这些东西缓存起来。为后续生成动态代理做准备。
使用getMapper方法生成动态代理
还是在MapperRegistry类中，调用getMapper方法生成动态代理类。关于动态代理不太熟悉的同学，可以相关设计模式，这个模式非常重要。
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 {
// 生成动态代理
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
再来追踪下MapperProxyFactory，我把其中不太重要的部分去掉了
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
// 去掉了一些不太重要的部分代码
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
// 实际使用了JDK的动态代理，接口类型就是初始化时候放进knownMappers中的某一个。
// 具体类型是在getMapper的时候传入的。
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}
实际使用了JDK的动态代理，接口类型就是初始化时候放进knownMappers中的某一个。具体类型是在getMapper的时候传入的。具体我们再回顾下上面那个MyBatis的调用案例。
MyBatis的示意代码
public Student findStudentById(int studentId) {
SqlSession sqlSession = MyBatisUtil.getSqlSession();
try {
StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
Student student = studentMapper.queryByPrimaryKey(studentId);
return student;
} finally {
sqlSession.close();
}
}
从这边可以看到。在调用getMapper时，会传入StudentMapper接口的类型。MyBatis会为这个接口生成动态代理类。下面我们看看这个动态代理实际执行了什么操作。找到MapperProxy类
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;
}
// 重点关注这个invoke方法
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
// 去掉了一些不太重要的部分代码
final MapperMethod mapperMethod = cachedMapperMethod(method);// 这边先看看缓存
return mapperMethod.execute(sqlSession, args);
}
// 去掉了一些不太重要的部分代码
}
在业务代码中调用任何一个sql方法，比如selectByPrimaryKey()都会来到这个invoke方法中。我们继续追踪mapperMethod.execute方法
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;
}
看到上面这个方法，大家应该会明白了。到这边，所有的方法都会根据command.getType()，分化为insert，update，delete，select这4个元操作。
接下来的调用代码就不再这边继续分析了。感兴趣的同学可以继续更进。
