Mybatis plugin的使用及原理示例解析

package org.apache.ibatis.plugin;
public interface Interceptor {
  /**
  * intercept方法就是要进行拦截的时候要执行的方法
  */
  Object intercept(Invocation invocation) throws Throwable;
  default Object plugin(Object target) {
    return Plugin.wrap(target, this);
  }
  default void setProperties(Properties properties) {
    // NOP
  }
}

@Component
@Intercepts({ @Signature(
        type = StatementHandler.class,
        method = "prepare",
        args = { Connection.class, Integer.class }) })
public class ExamplePlugin implements Interceptor {
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        StatementHandler statementHandler = (StatementHandler)invocation.getTarget();
        BoundSql boundSql = statementHandler.getBoundSql();
        String sql = boundSql.getSql();
        String newSql = null;
        // 改造sql部分省略，主要是将 instr() 拆分成 instr() or instr() 的形式以降低每个括号内	的长度...
        // newsql = sql.replace(...)
        // 把新的sql通过反射重新设置回去
        Field field = boundSql.getClass().getDeclaredField("sql");
        field.setAccessible(true);
        field.set(boundSql , newSql);
        Object returnVal = invocation.proceed();
        return returnVal;
    }
}

@Intercepts({
        @Signature(
                type = ResultSetHandler.class,
                method = "handleResultSets", 
                args = {Statement.class}),
        @Signature(type = Executor.class,
                method = "query",
                args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})
})

@Intercepts({ @Signature(
        type = StatementHandler.class,
        method = "prepare",
        args = { Connection.class, Integer.class }) })

@org.springframework.context.annotation.Configuration
@ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class })
@ConditionalOnSingleCandidate(DataSource.class)
@EnableConfigurationProperties(MybatisProperties.class)
@AutoConfigureAfter({ DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class })
public class MybatisAutoConfiguration implements InitializingBean {
  // 省略部分代码
  public MybatisAutoConfiguration(MybatisProperties properties, ObjectProvider<Interceptor[]> interceptorsProvider,
      ObjectProvider<TypeHandler[]> typeHandlersProvider, ObjectProvider<LanguageDriver[]> languageDriversProvider,
      ResourceLoader resourceLoader, ObjectProvider<DatabaseIdProvider> databaseIdProvider,
      ObjectProvider<List<ConfigurationCustomizer>> configurationCustomizersProvider) {
    this.properties = properties;
    this.interceptors = interceptorsProvider.getIfAvailable();
    this.typeHandlers = typeHandlersProvider.getIfAvailable();
    this.languageDrivers = languageDriversProvider.getIfAvailable();
    this.resourceLoader = resourceLoader;
    this.databaseIdProvider = databaseIdProvider.getIfAvailable();
    this.configurationCustomizers = configurationCustomizersProvider.getIfAvailable();
  }
}

<configuration>    
	<plugins>
        <plugin interceptor="com.zhanfu.spring.demo.utils.ExamplePlugin"/>
    </plugins>
</configuration>

public class InterceptorChain {
  // 所有的插件都存在这个 List 中
  private final List<Interceptor> interceptors = new ArrayList<>();
  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }
  public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
  }
  public List<Interceptor> getInterceptors() {
    return Collections.unmodifiableList(interceptors);
  }
}

  // Interceptor.java
  default Object plugin(Object target) {
    return Plugin.wrap(target, this);
  }
  // Plugin.java
  public static Object wrap(Object target, Interceptor interceptor) {
    // 从插件的注解中，解析出该插件可作用的接口，以及该类下的哪些方法
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();
    // 找到插件可作用的接口和目标类的中所有重合的接口
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    // 如果有重合的接口，则生成jdk代理并返回。注意，interceptor是我们的插件对象，signatureMap是插件注释解析到的类与方法
    if (interfaces.length > 0) {
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));
    }
    // 如果没有，则返回原对象
    return target;
  }

  // Plugin.java
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      // 只有注解上指定方法才能走插件对象的 intercept 方法
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
      if (methods != null && methods.contains(method)) {
        return interceptor.intercept(new Invocation(target, method, args));
      }
      // 其他方法尽管经过代理，但其实什么也没做，直接调用原对象去了
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }