首先现在已经有很多Mybatis源码分析的文章，之所以重复造轮子，只是为了督促自己更好的理解源码。

 

1.先看一段PageHelper拦截器的配置,在mybatis的配置文件<configuration>标签下配置。

1.1 其它的PageHelper的属性配置以及默认值可以参考com.github.pagehelper.page.PageParams类

public class PageParams {    //RowBounds参数offset作为PageNum使用 - 默认不使用    protected boolean offsetAsPageNum = false;    //RowBounds是否进行count查询 - 默认不查询    protected boolean rowBoundsWithCount = false;    //当设置为true的时候，如果pagesize设置为0（或RowBounds的limit=0），就不执行分页，返回全部结果    protected boolean pageSizeZero = false;    //分页合理化    protected boolean reasonable = false;    //是否支持接口参数来传递分页参数，默认false    protected boolean supportMethodsArguments = false;    //默认count(0)    protected String countColumn = "0";     public void setProperties(Properties properties) {        //offset作为PageNum使用        String offsetAsPageNum = properties.getProperty("offsetAsPageNum");        this.offsetAsPageNum = Boolean.parseBoolean(offsetAsPageNum);        //RowBounds方式是否做count查询        String rowBoundsWithCount = properties.getProperty("rowBoundsWithCount");        this.rowBoundsWithCount = Boolean.parseBoolean(rowBoundsWithCount);        //当设置为true的时候，如果pagesize设置为0（或RowBounds的limit=0），就不执行分页        String pageSizeZero = properties.getProperty("pageSizeZero");        this.pageSizeZero = Boolean.parseBoolean(pageSizeZero);        //分页合理化，true开启，如果分页参数不合理会自动修正。默认false不启用        String reasonable = properties.getProperty("reasonable");        this.reasonable = Boolean.parseBoolean(reasonable);        //是否支持接口参数来传递分页参数，默认false        String supportMethodsArguments = properties.getProperty("supportMethodsArguments");        this.supportMethodsArguments = Boolean.parseBoolean(supportMethodsArguments);        //默认count列        String countColumn = properties.getProperty("countColumn");        if(StringUtil.isNotEmpty(countColumn)){            this.countColumn = countColumn;        }        //当offsetAsPageNum=false的时候，不能        //参数映射        PageObjectUtil.setParams(properties.getProperty("params"));    }}

 

1.2 添加配置后自定义的拦截器需要增加注解@Intercepts否则会报错

@Intercepts(        {
    　　　　　　　　　　//拦截excutor类型里面的query方法                @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),                @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class}),        })public class PageInterceptor implements Interceptor {    private volatile Dialect dialect;    private String countSuffix = "_COUNT";    protected Cache
     
       msCountMap = null;    private String default_dialect_class = "com.github.pagehelper.PageHelper";}
     

private static Map
     
      , Set
      
       > getSignatureMap(Interceptor interceptor) {    Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);    // issue #251    //查看拦截器是否有Intercepts注解 没有抛出异常    if (interceptsAnnotation == null) {      throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());    }    Signature[] sigs = interceptsAnnotation.value();    Map
       
        , Set
        
         > signatureMap = new HashMap<>();    for (Signature sig : sigs) {      Set
         
           methods = signatureMap.computeIfAbsent(sig.type(), k -> new HashSet<>());      try {        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;  }
         
        
       
      
     

 

1.3使用PageHelper

1 public static void main(String[] args) throws IOException { 2  3         InputStream is = Resources.getResourceAsStream("config/mybatis-config.xml"); 4         SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);//DefaultSqlSessionFactory 5  6         SqlSession session = sqlSessionFactory.openSession();//打开session会执行 InterceptorChina.plugAll() --->Plugin.wrap()生成代理对象,被代理的就是Excutor 7          8         IStudentDao sudentDaoProxy = session.getMapper(IStudentDao.class); 9 　　　　　//加上这句就会拦截查询方法并进行分页10         Page page = PageHelper.startPage(1,1,"name");11         List
     
       student = sudentDaoProxy.findStudentById(null);12     }
     

 

 

2.第一部分是如何使用拦截器，第二部分则是拦截器如何执行的。Mybatis插件的核心接口是 org.apache.ibatis.plugin.Interceptor。

public interface Interceptor {  //拦截器执行业务逻辑方法    Object intercept(Invocation invocation) throws Throwable;  //设置拦截的真实对象  Object plugin(Object target);  //设置拦截器初始化属性  void setProperties(Properties properties);}

2.1.XMLConfigBuilder类解析xml所有配置包括<Plugins>标签，然后放Mybatis全局配置类Configuration中

//XMLConfigBuilder类 private void pluginElement(XNode parent) throws Exception {    //如果有Plugins标签    if (parent != null) {     //拿到所有的子标签也就是
     
      ，所有的拦截器配置      for (XNode child : parent.getChildren()) {        //拦截器的类        String interceptor = child.getStringAttribute("interceptor");        //
      
       配置        Properties properties = child.getChildrenAsProperties();        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();        //把property属性设置到拦截器里面        interceptorInstance.setProperties(properties);       //加入到拦截器链中List
       
         interceptors        configuration.addInterceptor(interceptorInstance);      }    }  }
       
      
     

2.2.解析完Configuration后生成DefaultSqlSessionFactory,打开session执行pluginAll

//DefaultSqlSessionFactoryprivate 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);     //选择excutor没有配置默认使用SimpleExecutor      final Executor executor = configuration.newExecutor(tx, execType);      return new DefaultSqlSession(configuration, executor, autoCommit);    } 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();    }  }

newExecutor() 

//Configuration 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);    }        //给Executor生成代理对象    executor = (Executor) interceptorChain.pluginAll(executor);    return executor;  }

//PageInterceptorpublic Object plugin(Object target) {        return Plugin.wrap(target, this);   }public class Plugin implements InvocationHandler {
    　　//生成Plugin代理对象   public static Object wrap(Object target, Interceptor interceptor) {    //拿到PageInterceptor类型的需要拦截的方法即query方法    Map
     
      , Set
      
       > signatureMap = getSignatureMap(interceptor);    Class
        type = target.getClass();    Class
       [] interfaces = getAllInterfaces(type, signatureMap);    if (interfaces.length > 0) {     //生成Plugin代理对象，所以当后面执行executor的query时其实执行下面的invoke方法      return Proxy.newProxyInstance(          type.getClassLoader(),          interfaces,          new Plugin(target, interceptor, signatureMap));    }    return target;  }@Override  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {    try {      Set
       
         methods = signatureMap.get(method.getDeclaringClass());      if (methods != null && methods.contains(method)) {      //执行PageInterceptor的intercept方法        return interceptor.intercept(new Invocation(target, method, args));      }      return method.invoke(target, args);    } catch (Exception e) {      throw ExceptionUtil.unwrapThrowable(e);    }  }}
       
      
     

2.3.已上就是为什么配置了拦截器后会执行拦截器的调用流程，本质上就是给Executor类型生成Plugin代理对象，以后executor执行的query方法通过plugin的invoke方法执行，invoke方法会调用自定义拦截器的intercept()方法。

2.4.所以PagerHelper分页现在需要做2件事情:1.拿到executor的查询sql变成 Select count(0) from table 形式查询count值；2.通过前面的coun值计算分页边界生成select * from table limit ?,?查询结果;

 

3. PagerHelper中的拦截方法比较清晰

public class PageInterceptor implements Interceptor {    @Override    public Object intercept(Invocation invocation) throws Throwable {        try {            Object[] args = invocation.getArgs();            MappedStatement ms = (MappedStatement) args[0];            Object parameter = args[1];            RowBounds rowBounds = (RowBounds) args[2];            ResultHandler resultHandler = (ResultHandler) args[3];            Executor executor = (Executor) invocation.getTarget();            CacheKey cacheKey;            BoundSql boundSql;            //由于逻辑关系，只会进入一次            if (args.length == 4) {                //4 个参数时                boundSql = ms.getBoundSql(parameter);                cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);            } else {                //6 个参数时                cacheKey = (CacheKey) args[4];                boundSql = (BoundSql) args[5];            }            checkDialectExists();            List resultList;            //调用方法判断是否需要进行分页，如果不需要，直接返回结果            if (!dialect.skip(ms, parameter, rowBounds)) {                //判断是否需要进行 count 查询                if (dialect.beforeCount(ms, parameter, rowBounds)) {                    //查询总数                    Long count = count(executor, ms, parameter, rowBounds, resultHandler, boundSql);                    //处理查询总数，返回 true 时继续分页查询，false 时直接返回                    if (!dialect.afterCount(count, parameter, rowBounds)) {                        //当查询总数为 0 时，直接返回空的结果                        return dialect.afterPage(new ArrayList(), parameter, rowBounds);                    }                }                resultList = ExecutorUtil.pageQuery(dialect, executor,                        ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);            } else {                //rowBounds用参数值，不使用分页插件处理时，仍然支持默认的内存分页                resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);            }            return dialect.afterPage(resultList, parameter, rowBounds);        } finally {            dialect.afterAll();        }    }}

通过ExecutorUtil.executeAutoCount()生成对应的countSql ----->经过了很多步到达CountSqlParser.sqlToCount()

public void sqlToCount(Select select, String name) {        SelectBody selectBody = select.getSelectBody();        // 是否能简化count查询        List
     
       COUNT_ITEM = new ArrayList
      
       ();        COUNT_ITEM.add(new SelectExpressionItem(new Column("count(" + name +")")));        if (selectBody instanceof PlainSelect && isSimpleCount((PlainSelect) selectBody)) {            //selectItems 就是sql中的select xxxx from table；把xxxx替换成count(0)            ((PlainSelect) selectBody).setSelectItems(COUNT_ITEM);        } else {            PlainSelect plainSelect = new PlainSelect();            SubSelect subSelect = new SubSelect();            subSelect.setSelectBody(selectBody);            subSelect.setAlias(TABLE_ALIAS);            plainSelect.setFromItem(subSelect);            plainSelect.setSelectItems(COUNT_ITEM);            select.setSelectBody(plainSelect);        }    }
      
     

 

3.1 ExecutorUtil.pageQuery()分页查询

public static  
     
       List
      
        pageQuery(Dialect dialect, Executor executor, MappedStatement ms, Object parameter,                                 RowBounds rowBounds, ResultHandler resultHandler,                                 BoundSql boundSql, CacheKey cacheKey) throws SQLException {        //判断是否需要进行分页查询        if (dialect.beforePage(ms, parameter, rowBounds)) {            //生成分页的缓存 key            CacheKey pageKey = cacheKey;            //处理参数对象            parameter = dialect.processParameterObject(ms, parameter, boundSql, pageKey);            //调用方言获取分页 sql            String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);            BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);            Map
       
         additionalParameters = getAdditionalParameter(boundSql);            //设置动态参数            for (String key : additionalParameters.keySet()) {                pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));            }            //执行分页查询            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);        } else {            //不执行分页的情况下，也不执行内存分页            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);        }    }