Mybatis：执行一个Sql命令的完整流程

Mybatis中的Sql命令，在枚举类SqlCommandType中定义的。

public enum SqlCommandType {
UNKNOWN, INSERT, UPDATE, DELETE, SELECT, FLUSH;
}

下面，我们以Mapper接口中的一个方法作为例子，看看Sql命令的执行完整流程。

public interface StudentMapper {
List<Student> findAllStudents(Map<String, Object> map, RowBounds rowBounds, ResultSetHandler rh);
}

参数RowBounds和ResultSetHandler是可选参数，表示分页对象和自定义结果集处理器，一般不需要。

一个完整的Sql命令，其执行的完整流程图如下：

上面的流程图完整展示了其执行流程。

MapperProxy的功能：

1. 因为Mapper接口不能直接实例化，MapperProxy的作用，就是使用JDK动态代理功能，间接实例化Mapper的proxy对象。

2. 缓存MapperMethod对象。

private final Map<Method, MapperMethod> methodCache;
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
//
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}

// 缓存MapperMethod
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}

MapperMethod的功能：

1. 解析Mapper接口的方法，并封装成MapperMethod对象。
2. 将Sql命令，正确路由到恰当的SqlSession的方法上。

public class MapperMethod {

// 保存了Sql命令的类型和键id
private final SqlCommand command;
// 保存了Mapper接口方法的解析信息
private final MethodSignature method;

public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
this.command = new SqlCommand(config, mapperInterface, method);
this.method = new MethodSignature(config, method);
}

// 根据解析结果，路由到恰当的SqlSession方法上
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
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()) {
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 {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
} else if (SqlCommandType.FLUSH == command.getType()) {
result = sqlSession.flushStatements();
} 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;
}
// ...

org.apache.ibatis.binding.MapperMethod.SqlCommand：

public static class SqlCommand {
// full id, 通过它可以找到MappedStatement
private final String name;
private final SqlCommandType type;
// ...

org.apache.ibatis.binding.MapperMethod.MethodSignature :

public static class MethodSignature {

private final boolean returnsMany;
private final boolean returnsMap;
private final boolean returnsVoid;
private final Class<?> returnType;
private final String mapKey;
private final Integer resultHandlerIndex;
private final Integer rowBoundsIndex;
private final SortedMap<Integer, String> params;
private final boolean hasNamedParameters;

public MethodSignature(Configuration configuration, Method method) {
this.returnType = method.getReturnType();
this.returnsVoid = void.class.equals(this.returnType);
this.returnsMany = (configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray());
this.mapKey = getMapKey(method);
this.returnsMap = (this.mapKey != null);
this.hasNamedParameters = hasNamedParams(method);
// 分页参数
this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
// 自定义ResultHandler
this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
this.params = Collections.unmodifiableSortedMap(getParams(method, this.hasNamedParameters));
}

以上是对MapperMethod的补充说明。
MapperMethod采用命令模式运行，根据上下文跳转，可能跳转到许多种方法中。

本节的重点，是上面的那个Sql命令完整执行流程图。如果不是使用Mapper接口调用，而是直接调用SqlSession的方法，那么，流程图从SqlSession的地方开始即可，后续都是一样的。