MyBatis的擴展點(plugins)

時間 2019-11-11

原文原文鏈接

一、mybatis擴展點plugins

mybatis的擴展是經過攔截器Interceptor來實現的，本質上就是JDK的動態代理，因此它只能對接口進行攔截，mybatis能夠對如下四個接口類型進行攔截，也就是說會對這4種對象進行代理，全部的代理攔截都是經過 InterceptorChain.pluginAll(Object target) 來實現的。html

Executor：執行器，執行SQL語句（全部的sql都經過它來執行），而且對事務、緩存等提供統一接口。（在這一層上作攔截的權限會更大）
StatementHandler：對statement進行預處理，而且提供統一的原子的增、刪、改、查接口。（若是要在SQL執行前進行攔截的話，攔截這裏就能夠了）
ResultSetHandler：對返回結果ResultSet進行處理。
PameterHandler：對參數進行賦值。java

SqlSession的建立過程：sql

mybatis中的SQL都是經過DefaultSqlSession去執行的，其建立過程以下：緩存

// SqlSessionFactoryBuilder => DefaultSqlSessionFactory => DefaultSqlSession
  SqlSession sqlSession = new SqlSessionFactoryBuilder().build(in, "development", pro).openSession();

二、源碼解讀具體實現（以Executor接口爲例）

2.一、建立SqlSession時，SqlSessionFactroy會解析mybatis.xml配置文件中的plugins標籤，並將Interceptor屬性定義的Interceptor放到interceptorChain中；session

// SqlSessionFactoryBuilder.java  
  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      // 解析mybatis.xml配置文件，並建立DefaultSqlSessionFactory
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        reader.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

// XMLConfigBuilder.java
  public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }
  // 解析mybatis.xml中的各個標籤
  private void parseConfiguration(XNode root) {
    try {
      propertiesElement(root.evalNode("properties")); //issue #117 read properties first
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      settingsElement(root.evalNode("settings"));
      environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }
  // 解析plugins標籤，並把Interceptor放到interceptorChain中
  private void pluginElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        String interceptor = child.getStringAttribute("interceptor");
        Properties properties = child.getChildrenAsProperties();
        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
        interceptorInstance.setProperties(properties);
        configuration.addInterceptor(interceptorInstance);
      }
    }
  }
  // Configuration，mybatis文件的抽象類
  public void addInterceptor(Interceptor interceptor) {
    interceptorChain.addInterceptor(interceptor);
  }

2.二、DefaultSqlSessionFactory.openSession()時使用JDK動態代理生成@Signature註解指定的被代理類（包含代理的方法以及方法參數） mybatis

// DefaultSqlSessionFactory.java
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      // 使用Configuration建立Executor
      final Executor executor = configuration.newExecutor(tx, execType, autoCommit);
      return new DefaultSqlSession(configuration, executor);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

// Configuration.java
  public Executor newExecutor(Transaction transaction, ExecutorType executorType, boolean autoCommit) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor, autoCommit);
    }
    // 使用JDK動態代理生成Executor的interceptorChain
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

2.三、InterceptorChain生成的具體過程app

// InterceptorChain.java
  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

// Interceptor的具體實現類（即咱們業務上要實現的功能）
  @Override
  public Object plugin(Object arg0) {
    return Plugin.wrap(arg0, this);
  }

// Plugin.java
  public static Object wrap(Object target, Interceptor interceptor) {
    // getSignatureMap獲取Interceptor類上的@Intercepts(@Signature)內容
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    // 生成目標類target(Executor.class)的代理類，實現咱們須要的plugin功能
    if (interfaces.length > 0) {
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));
    }
    return target;
  }

  // 解析實現Interceptor接口的類上定義的@Intercepts(@Signature)內容，獲取須要攔截的類和方法。
  // 例如：@Signature(type = Executor.class, method = "query",  args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})
  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
    Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
    if (interceptsAnnotation == null) { // issue #251
      throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());      
    }
    Signature[] sigs = interceptsAnnotation.value();
    Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
    for (Signature sig : sigs) {
      Set<Method> methods = signatureMap.get(sig.type());
      if (methods == null) {
        methods = new HashSet<Method>();
        signatureMap.put(sig.type(), methods);
      }
      try {
        // sig.type()即Executor.class
        Method method = sig.type().getMethod(sig.method(), sig.args());
        methods.add(method);
      } catch (NoSuchMethodException e) {
        throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
      }
    }
    return signatureMap;
  }

三、demo關鍵步驟

3.一、實現自定義的Interceptoride

// 自定義攔截器
  @Intercepts({@Signature(type = Executor.class, method = "query",  
	 args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})
  public class MyTestInterceptor implements Interceptor {
    private static final String MSG = "octopus route table info is not exit!";

	@Override
	public Object intercept(Invocation arg0) throws Throwable {
		Object obj = null;
		try {
			obj = arg0.proceed();
		} catch (Throwable e) {
			if (e.getCause() instanceof MySQLSyntaxErrorException) {
				MySQLSyntaxErrorException ex = (MySQLSyntaxErrorException) e.getCause();
				System.out.println("====" + ex.getErrorCode());
				System.out.println("====" + ex.getSQLState());
				System.out.println("====" + ex.getMessage());
				System.out.println("====" + ex.getCause());
				if (MSG.equals(ex.getMessage())) {
					throw new RouteTableNoExistException();
				}
			}
		}
		return obj;
	}

	@Override
	public Object plugin(Object arg0) {
		return Plugin.wrap(arg0, this);
	}

	@Override
	public void setProperties(Properties arg0) {
		System.out.println("env value: " + arg0.getProperty("names"));
	}

  }

3.二、在mybatis.xml中配置pluginsfetch

<configuration>
	<plugins>
		<plugin interceptor="com.pinganfu.interceptor.MyTestInterceptor" />
	</plugins>	

	<environments default="development">
		<environment id="development">
			<transactionManager type="MANAGED">
				<property name="closeConnection" value="false" />
			</transactionManager>
			<dataSource type="POOLED">
				<property name="driver" value="${driver}" />
				<property name="url" value="${jdbcUrl}" />
				<property name="username" value="${username}" />
				<property name="password" value="${password}" />
			</dataSource>
		</environment>
	</environments>	

	<mappers>
		<mapper resource="mappers/TBATMapper.xml" />
	</mappers>	
</configuration>

3.三、獲取SqlSessionui

Properties pro = new Properties();
  try {
    pro.load(Resources.getResourceAsStream("jdbc.properties"));
    // 加載mybatis.xml中的plugins
	InputStream in = Resources.getResourceAsStream("mybatis.xml");
	sqlSession = new SqlSessionFactoryBuilder().build(in, "development", pro).openSession();
  } catch (IOException e) {
	e.printStackTrace();
  }

四、mybatis針對各類異常的處理

mybatis經過DefaultSqlSession執行時，會將發生的全部異常統一包裝成PersistenceException再拋出，咱們能夠經過PersistenceException.getCause()獲取具體的異常。

// DefaultSqlSession.java
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
      return result;
    } catch (Exception e) {
      // 對執行發生的全部Exception進行wrap以後再拋出
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

// ExceptionFactory.java
  public static RuntimeException wrapException(String message, Exception e) {
    // 將Exception進行統一包裝成PersistenceException
    return new PersistenceException(ErrorContext.instance().message(message).cause(e).toString(), e);
  }

Ref：

https://www.cnblogs.com/kevin-yuan/p/7219003.html

https://www.cnblogs.com/fangjian0423/p/mybatis-interceptor.html

https://www.cnblogs.com/fenglanglang/p/6007653.html

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