Mybatis插件机制以及PageHelper插件的原理
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。
<plugins>
<plugin interceptor="com.github.pagehelper.PageInterceptor">
<!--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-->
<property name="pageSizeZero" value="false"/>
</plugin>
</plugins>
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<String, MappedStatement> msCountMap = null; private String default_dialect_class = "com.github.pagehelper.PageHelper"; }
private static Map<Class<?>, Set<Method>> 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<Class<?>, Set<Method>> signatureMap = new HashMap<>(); for (Signature sig : sigs) { Set<Method> 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> 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<Plugin>，所有的拦截器配置 for (XNode child : parent.getChildren()) { //拦截器的类 String interceptor = child.getStringAttribute("interceptor"); //<Property>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<Interceptor> interceptors configuration.addInterceptor(interceptorInstance); } } }
2.2.解析完Configuration后生成DefaultSqlSessionFactory,打开session执行pluginAll
//DefaultSqlSessionFactory 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); //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; }
//PageInterceptor public 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<Class<?>, Set<Method>> 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<Method> 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<SelectItem> COUNT_ITEM = new ArrayList<SelectItem>(); 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 <E> List<E> 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<String, Object> 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); } }
