Analysis of the execution process of Mapper interface of MyBatis

Mabtis, as an excellent persistence framework, not only improves our development efficiency, but also has the characteristics of convenience and rapidity. Usually when we configure MyBatis, we will configure a series of Mappers interface classes to MyBatis. However, if we do not configure these Mappers, we can also execute SQL statements in Mapper.xml file. The reason for configuration is that it is more convenient to use interfaces directly than not. Let's see how to execute SQL without interfaces.

Mapper.xml is as follows:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
  
<mapper namespace="org.mapper.UserMapper">
	<cache/>
    <select id="selectAll"  resultType="User">
        select * from tb_oder
    </select>
    <select id="selectByName"  resultType="User" parameterType="string"
    	useCache="false">
        select * from tb_oder where name like concat(#{keyWords},'%')
    </select>
    <update id="updateAge" parameterType="User" flushCache="true">
    	update tb_oder set age=#{age} where name =#{name}
    </update>
</mapper>

Some related configurations will not be described in detail. Please check the previous blog if necessary. Now we need to call the statement with the ID of SelectAll in the XML. We just need to execute the session.selectList("selectAll") statement after obtaining the session. The logic of the method is divided into two steps: the first step is to find the corresponding MappedStatement according to the configuration of selectAll to MyBatis, then invoke the query method of the executor, and pass the MappedStatement querying on the previous step as the parameter, and finally return the result of the query method of the actuator. Add: mappedstatement is stored in a Map in configuration when configuration is loaded. Each mappedstatement has an ID, which is the ID of SQL statement in mapper.xml. If mapper.xml is configured with namspace, the ID will change to the ID of the namspace.sql statement.

@Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

Next, let's see how to operate the database through the configured mapper interface?

1. Get the specified Mapper

The following methods in DefaultSqlSession:

@Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

The getMapper of Configuration is as follows:

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }
@SuppressWarnings("unchecked")
  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 {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

The key is in the last getMapper method. We gradually analyze:

1. What are knownmappers?

It is a final private class variable of MapperRegistry, pointing to the object address of a HashMap.

public class MapperRegistry {

  private final Configuration config;
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

  public MapperRegistry(Configuration config) {
    this.config = config;
  }

  @SuppressWarnings("unchecked")
  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 {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }
  
  public <T> boolean hasMapper(Class<T> type) {
    return knownMappers.containsKey(type);
  }

  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.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.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

  /**
   * @since 3.2.2
   */
  public Collection<Class<?>> getMappers() {
    return Collections.unmodifiableCollection(knownMappers.keySet());
  }

  /**
   * @since 3.2.2
   */
  public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
  }

  /**
   * @since 3.2.2
   */
  public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
  }
  
}

2. final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type)

This is to get mapperproxyfactory of the specified type from knownMappers. So what is mapperproxyfactory? According to the naming convention, you can know a project class, which is used to create MapperProxy in the factory. Next, let's look at mapperproxyfactory, MapperProxy and MapperMethod.

2.1 MapperProxyFactory

public class MapperProxyFactory<T> {

  private final Class<T> mapperInterface;
  private final 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) {
    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);
  }

}

2.2 MapperProxy

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;
  }

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

  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;
  }

  @UsesJava7
  private Object invokeDefaultMethod(Object proxy, Method method, Object[] args)
      throws Throwable {
    final Constructor<MethodHandles.Lookup> constructor = MethodHandles.Lookup.class
        .getDeclaredConstructor(Class.class, int.class);
    if (!constructor.isAccessible()) {
      constructor.setAccessible(true);
    }
    final Class<?> declaringClass = method.getDeclaringClass();
    return constructor
        .newInstance(declaringClass,
            MethodHandles.Lookup.PRIVATE | MethodHandles.Lookup.PROTECTED
                | MethodHandles.Lookup.PACKAGE | MethodHandles.Lookup.PUBLIC)
        .unreflectSpecial(method, declaringClass).bindTo(proxy).invokeWithArguments(args);
  }

  /**
   * Backport of java.lang.reflect.Method#isDefault()
   */
  private boolean isDefaultMethod(Method method) {
    return (method.getModifiers()
        & (Modifier.ABSTRACT | Modifier.PUBLIC | Modifier.STATIC)) == Modifier.PUBLIC
        && method.getDeclaringClass().isInterface();
  }
}
public class MapperMethod {

  private final SqlCommand command;
  private final MethodSignature method;

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

  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;
  }

  private Object rowCountResult(int rowCount) {
    final Object result;
    if (method.returnsVoid()) {
      result = null;
    } else if (Integer.class.equals(method.getReturnType()) || Integer.TYPE.equals(method.getReturnType())) {
      result = rowCount;
    } else if (Long.class.equals(method.getReturnType()) || Long.TYPE.equals(method.getReturnType())) {
      result = (long)rowCount;
    } else if (Boolean.class.equals(method.getReturnType()) || Boolean.TYPE.equals(method.getReturnType())) {
      result = rowCount > 0;
    } else {
      throw new BindingException("Mapper method '" + command.getName() + "' has an unsupported return type: " + method.getReturnType());
    }
    return result;
  }

  private void executeWithResultHandler(SqlSession sqlSession, Object[] args) {
    MappedStatement ms = sqlSession.getConfiguration().getMappedStatement(command.getName());
    if (!StatementType.CALLABLE.equals(ms.getStatementType())
        && void.class.equals(ms.getResultMaps().get(0).getType())) {
      throw new BindingException("method " + command.getName()
          + " needs either a @ResultMap annotation, a @ResultType annotation,"
          + " or a resultType attribute in XML so a ResultHandler can be used as a parameter.");
    }
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      sqlSession.select(command.getName(), param, rowBounds, method.extractResultHandler(args));
    } else {
      sqlSession.select(command.getName(), param, method.extractResultHandler(args));
    }
  }

  private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
    List<E> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<E>selectList(command.getName(), param, rowBounds);
    } else {
      result = sqlSession.<E>selectList(command.getName(), param);
    }
    // issue #510 Collections & arrays support
    if (!method.getReturnType().isAssignableFrom(result.getClass())) {
      if (method.getReturnType().isArray()) {
        return convertToArray(result);
      } else {
        return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
      }
    }
    return result;
  }

  private <T> Cursor<T> executeForCursor(SqlSession sqlSession, Object[] args) {
    Cursor<T> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<T>selectCursor(command.getName(), param, rowBounds);
    } else {
      result = sqlSession.<T>selectCursor(command.getName(), param);
    }
    return result;
  }

  private <E> Object convertToDeclaredCollection(Configuration config, List<E> list) {
    Object collection = config.getObjectFactory().create(method.getReturnType());
    MetaObject metaObject = config.newMetaObject(collection);
    metaObject.addAll(list);
    return collection;
  }

  @SuppressWarnings("unchecked")
  private <E> Object convertToArray(List<E> list) {
    Class<?> arrayComponentType = method.getReturnType().getComponentType();
    Object array = Array.newInstance(arrayComponentType, list.size());
    if (arrayComponentType.isPrimitive()) {
      for (int i = 0; i < list.size(); i++) {
        Array.set(array, i, list.get(i));
      }
      return array;
    } else {
      return list.toArray((E[])array);
    }
  }

  private <K, V> Map<K, V> executeForMap(SqlSession sqlSession, Object[] args) {
    Map<K, V> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey(), rowBounds);
    } else {
      result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey());
    }
    return result;
  }

  public static class ParamMap<V> extends HashMap<String, V> {

    private static final long serialVersionUID = -2212268410512043556L;

    @Override
    public V get(Object key) {
      if (!super.containsKey(key)) {
        throw new BindingException("Parameter '" + key + "' not found. Available parameters are " + keySet());
      }
      return super.get(key);
    }

  }

  public static class SqlCommand {

    private final String name;
    private final SqlCommandType type;

    public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
      final String methodName = method.getName();
      final Class<?> declaringClass = method.getDeclaringClass();
      MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass,
          configuration);
      if (ms == null) {
        if (method.getAnnotation(Flush.class) != null) {
          name = null;
          type = SqlCommandType.FLUSH;
        } else {
          throw new BindingException("Invalid bound statement (not found): "
              + mapperInterface.getName() + "." + methodName);
        }
      } else {
        name = ms.getId();
        type = ms.getSqlCommandType();
        if (type == SqlCommandType.UNKNOWN) {
          throw new BindingException("Unknown execution method for: " + name);
        }
      }
    }

    public String getName() {
      return name;
    }

    public SqlCommandType getType() {
      return type;
    }

    private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName,
        Class<?> declaringClass, Configuration configuration) {
      String statementId = mapperInterface.getName() + "." + methodName;
      if (configuration.hasStatement(statementId)) {
        return configuration.getMappedStatement(statementId);
      } else if (mapperInterface.equals(declaringClass)) {
        return null;
      }
      for (Class<?> superInterface : mapperInterface.getInterfaces()) {
        if (declaringClass.isAssignableFrom(superInterface)) {
          MappedStatement ms = resolveMappedStatement(superInterface, methodName,
              declaringClass, configuration);
          if (ms != null) {
            return ms;
          }
        }
      }
      return null;
    }
  }

  public static class MethodSignature {

    private final boolean returnsMany;
    private final boolean returnsMap;
    private final boolean returnsVoid;
    private final boolean returnsCursor;
    private final Class<?> returnType;
    private final String mapKey;
    private final Integer resultHandlerIndex;
    private final Integer rowBoundsIndex;
    private final ParamNameResolver paramNameResolver;

    public MethodSignature(Configuration configuration, Class<?> mapperInterface, Method method) {
      Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface);
      if (resolvedReturnType instanceof Class<?>) {
        this.returnType = (Class<?>) resolvedReturnType;
      } else if (resolvedReturnType instanceof ParameterizedType) {
        this.returnType = (Class<?>) ((ParameterizedType) resolvedReturnType).getRawType();
      } else {
        this.returnType = method.getReturnType();
      }
      this.returnsVoid = void.class.equals(this.returnType);
      this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray();
      this.returnsCursor = Cursor.class.equals(this.returnType);
      this.mapKey = getMapKey(method);
      this.returnsMap = this.mapKey != null;
      this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
      this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
      this.paramNameResolver = new ParamNameResolver(configuration, method);
    }

    public Object convertArgsToSqlCommandParam(Object[] args) {
      return paramNameResolver.getNamedParams(args);
    }

    public boolean hasRowBounds() {
      return rowBoundsIndex != null;
    }

    public RowBounds extractRowBounds(Object[] args) {
      return hasRowBounds() ? (RowBounds) args[rowBoundsIndex] : null;
    }

    public boolean hasResultHandler() {
      return resultHandlerIndex != null;
    }

    public ResultHandler extractResultHandler(Object[] args) {
      return hasResultHandler() ? (ResultHandler) args[resultHandlerIndex] : null;
    }

    public String getMapKey() {
      return mapKey;
    }

    public Class<?> getReturnType() {
      return returnType;
    }

    public boolean returnsMany() {
      return returnsMany;
    }

    public boolean returnsMap() {
      return returnsMap;
    }

    public boolean returnsVoid() {
      return returnsVoid;
    }

    public boolean returnsCursor() {
      return returnsCursor;
    }

    private Integer getUniqueParamIndex(Method method, Class<?> paramType) {
      Integer index = null;
      final Class<?>[] argTypes = method.getParameterTypes();
      for (int i = 0; i < argTypes.length; i++) {
        if (paramType.isAssignableFrom(argTypes[i])) {
          if (index == null) {
            index = i;
          } else {
            throw new BindingException(method.getName() + " cannot have multiple " + paramType.getSimpleName() + " parameters");
          }
        }
      }
      return index;
    }

    private String getMapKey(Method method) {
      String mapKey = null;
      if (Map.class.isAssignableFrom(method.getReturnType())) {
        final MapKey mapKeyAnnotation = method.getAnnotation(MapKey.class);
        if (mapKeyAnnotation != null) {
          mapKey = mapKeyAnnotation.value();
        }
      }
      return mapKey;
    }
  }

}

Continue to analyze the previous sentence: mapperproxyfactory. New instance (sqlsession). The view implementation is as follows:

@SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    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);
  }

First, generate a MapperProxy, then call the overloaded newInstance method to generate a proxy object. This is the key for Mapper interface to implement SQL execution, based on the dynamic agent mechanism of JDK. Next, let's look at mapperproxy's code, overriding the invoke() method,

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

In the implementation of this method, we can see the last line mapperMethod.execute(sqlSession, args). After checking the implementation, we can see the following code:

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;
  }

We can see that the corresponding methods in sqlSession are called according to different types of methods, and the subsequent execution logic is the same as that of MappedStatement at the beginning of this article. At this point, we already know how the Mapper interface executes. There are also some Mybatis configurations where Mapper is stored in knownMappers and a MapperProxyFactory is generated at the same time without any specific analysis.

Summary: the first stage: create a MapperProxyFactory for each Mapper during the configuration loading process; the second stage: obtain the Mapper implementation class to generate a proxy implementation class for each Mapper through the dynamic proxy of JDK and return MapperProxy; the third stage: call the method of the proxy class. In the invoke method of MapperProxy, mappermethod is generated according to the executed interface method name. If there is a cache, the cached mappermethod is used directly. Then we call the execute method of MapperMethod, which calls the corresponding method in session, and then finds the corresponding MappedStatement, then executes the sql corresponding to MappedStatement with the executor.

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Tags: Mybatis xml SQL Session

Posted on Sun, 02 Feb 2020 01:48:41 -0500 by Mastermind