Mybatis是如何执行一条SQL命令

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：

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。可参看系列的第二篇。

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); } } // 投鞭断流 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。