Mybatis拦截器并不是每个对象里面的方法都可以被拦截的。Mybatis拦截器只能拦截Executor、StatementHandler、ParameterHandler、ResultSetHandler四个类里面的方法，这四个对象在创建的时候才会创建代理。

用途：实际工作中，可以使用Mybatis拦截器来做一些SQL权限校验、数据过滤、数据加密脱敏、SQL执行时间性能监控和告警等。

 1.使用方法

以在Spring中创建 StatementHandler.update()方法的拦截器为例：

@Component
@Order(1)
@Intercepts({@Signature(type = StatementHandler.class, method = "update", args = {Statement.class}),})
public class SqlValidateMybatisInterceptor extends PRSMybatisInterceptor {
 
    @Override
    protected Object before(Invocation invocation) throws Throwable {
        String sql="";
        Statement statement=(Statement) invocation.getArgs()[0];
        if(Proxy.isProxyClass(statement.getClass())){
            MetaObject metaObject= SystemMetaObject.forObject(statement);
            Object h=metaObject.getValue("h");
            if(h instanceof StatementLogger){
                RoutingStatementHandler rsh=(RoutingStatementHandler) invocation.getTarget();
                sql=rsh.getBoundSql().getSql();
            }else {
                PreparedStatementLogger psl=(PreparedStatementLogger) h;
                sql=psl.getPreparedStatement().toString();
            }
        }else{
            sql=statement.toString();
        }
        if(containsDelete(sql)&&!containsWhere(sql)){
            throw new SQLException("不能删除整张表,sql:"+sql);
        }
        return null;
    }
 
    private boolean containsDelete(String sql){
        return sql.contains("delete")||sql.contains("DELETE");
    }
 
    private boolean containsWhere(String sql){
        return sql.contains("where")||sql.contains("WHERE");
    }
}
public class PRSMybatisInterceptor implements Interceptor {
 
    Boolean needBreak=false;
 
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        Object result= before(invocation);
        if(needBreak){
            return result;
        }
        result= invocation.proceed();
        result=after(result,invocation);
        return result;
    }
 
    protected Object before(Invocation invocation) throws Throwable{
        return null;
    }
    protected Object after(Object result,Invocation invocation) throws Throwable{
        return result;
    }
 
    @Override
    public Object plugin(Object o) {
        return Plugin.wrap(o, this);
    }
 
    @Override
    public void setProperties(Properties properties) {
    }
}

1. 自定义拦截器 实现 org.apache.ibatis.plugin.Interceptor 接口与其中的方法。在plugin方法中需要返回 return Plugin.wrap(o, this)。在intercept方法中可以实现拦截的业务逻辑，改方法的 参数 Invocation中有原始调用的 对象，方法和参数，可以对其任意处理。

2. 在自定义的拦截器上添加需要拦截的对象和方法，通过注解 org.apache.ibatis.plugin.Intercepts 添加。如示例代码所示：

Intercepts的值是一个签名数组，签名中包含要拦截的 类，方法和参数。

2.MyBatis对象的创建

代理对象指的是：可以被拦截的4个类的实例。

代理对象创建时需要解析拦截器，从而利用JDK动态代理将拦截器的逻辑织入原始对象。

DefaultSqlSession中依赖Executor，如果新建的时候会创建executor

private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) {
    ...
    final Executor executor = configuration.newExecutor(tx, execType);
    return new DefaultSqlSession(configuration, executor, autoCommit);
}
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
  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);
  }
  executor = (Executor) interceptorChain.pluginAll(executor);
  return executor;
}

Executor中要用StatementHandler执行sql语句，StatementHandler是调用configuration.newStatementHandler()方法创建的。

StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameterObject, rowBounds, resultHandler, boundSql);
 
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
  StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
  statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
  return statementHandler;
}

StatementHandler依赖 parameterHandler 和 resultSetHandler，在构造 StatementHandler 时会调用一下方法创建这两个 handler。

this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
  ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
  parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
  return parameterHandler;
}

this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
    ResultHandler resultHandler, BoundSql boundSql) {
  ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
  resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
  return resultSetHandler;
}

3.代理对象的创建

3.1 拦截器的获取

从对象的创建过程中可以看出 代理 对象的创建时通过 InterceptorChain.pluginAll() 方法创建的。

查看 拦截器链 InterceptorChain 发现，其中的拦截器的添加是在 Configuration 中。因为拦截器被声明为Bean了，所以在MyBatis初始化的时候，会扫描所有拦截器，添加到 InterceptorChain 中。

3.2 代理对象的创建

从上一步得知代理对象的创建是调用 Interceptor.pugin() 方法，然后调用 Plugin.wrap() 方法

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

Plugin实现了 InvocationHandler 接口

 在 Plugin.wrap() 方法中会获取当前拦截器的接口，生成动态代理。

4. 拦截器的执行过程

在动态代理中当代理对象调用方法时，会将方法的调用委托给 InvocationHandler，也就是 Plugin，如下图所示

 在该方法中 获取拦截器签名中的方法，如果包含当前方法，则调用拦截方法，否则执行原方法的调用。

5. 拦截器的执行顺序

拦截器的顺序配置使用 Spring 中的 org.springframework.core.annotation.Order 注解配置。

order值大的拦截器先执行，order值大的在interceptors中越靠后，最后生成代理，所以先执行。

6. 拦截器示例

此拦截器为一个通用的拦截器，封装了具体的拦截器实现，开放出来before和after方法供业务调用。

public class PRSMybatisInterceptor implements Interceptor {
    Boolean needBreak = false;
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        Object result = before(invocation);
        if (needBreak) {
            return result;
        }
        result = invocation.proceed();
        result = after(result, invocation);
        return result;
    }
    protected Object before(Invocation invocation) throws Throwable {
        return null;
    }
    protected Object after(Object result, Invocation invocation) throws Throwable {
        return result;
    }
    @Override
    public Object plugin(Object o) {
        return Plugin.wrap(o, this);
    }
    @Override
    public void setProperties(Properties properties) {
    }
}

/**
 * @author CaptHua
 */
@Component
@Order(1)
@Intercepts({@Signature(type = StatementHandler.class, method = "update", args = {Statement.class}),})
public class SqlValidateMybatisInterceptor extends PRSMybatisInterceptor {
    @Override
    protected Object before(Invocation invocation) throws Throwable {
        String sql = "";
        Statement statement = (Statement) invocation.getArgs()[0];
        if (Proxy.isProxyClass(statement.getClass())) {
            MetaObject metaObject = SystemMetaObject.forObject(statement);
            Object h = metaObject.getValue("h");
            if (h instanceof StatementLogger) {
                RoutingStatementHandler rsh = (RoutingStatementHandler) invocation.getTarget();
                sql = rsh.getBoundSql().getSql();
            } else {
                PreparedStatementLogger psl = (PreparedStatementLogger) h;
                sql = psl.getPreparedStatement().toString();
            }
        } else {
            sql = statement.toString();
        }
        if (containsDelete(sql) && !containsWhere(sql)) {
            throw new SQLException("不能删除整张表,sql:" + sql);
        }
        return null;
    }
    private boolean containsDelete(String sql) {
        return sql.contains("delete") || sql.contains("DELETE");
    }
    private boolean containsWhere(String sql) {
        return sql.contains("where") || sql.contains("WHERE");
    }
}