PageHelper 实现分页原理
一、PageHelper 的使用
在 SpringBoot 中引入对应的 starter:
<dependency>
<groupId>com.github.pagehelper</groupId>
<artifactId>pagehelper-spring-boot-starter</artifactId>
<version>1.3.0</version>
</dependency>
在查询数据时,在查询方法前添加上 PageHelper 的 start 方法即可:
@Test
public void select() {
PageHelper.startPage(1, 10);
List<Order> orders = orderMapper.queryAll();
log.info("记录条数为:{}", orders.size());
}
通过 PageHelper 分页插件,可以快速的帮助我们实现分页,我们不用在 sql 语句中使用 limit 来手动分页了。
二、PageHelper 实现原理
2.1 使用 ThreadLocal 记录分页参数
在调用 startPage
方法时,会通过 ThreadLocal 存储当前分页参数:
public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {
Page<E> page = new Page<E>(pageNum, pageSize, count);
page.setReasonable(reasonable);
page.setPageSizeZero(pageSizeZero);
//当已经执行过orderBy的时候
Page<E> oldPage = getLocalPage();
if (oldPage != null && oldPage.isOrderByOnly()) {
page.setOrderBy(oldPage.getOrderBy());
}
//设置ThreadLocal
setLocalPage(page);
return page;
}
2.2 使用 Mybatis 拦截器机制
这里先介绍一下 Mybatis 拦截器机制。
Mybatis 的拦截器机制是指用户可以自定义拦截器,通过代理的方式,增强 Mybatis 中的核心组件,目前只支持 4 个核心组件:ParameterHandler、ResultSetHandler、StatementHandler 和 Executor。
通过 Interceptor 接口可以实现自定义拦截器:
public interface Interceptor {
Object intercept(Invocation invocation) throws Throwable;
// 默认方法
default Object plugin(Object target) {
return Plugin.wrap(target, this);
}
default void setProperties(Properties properties) {
// NOP
}
}
在 Configuration 初始化配置时,XMLConfigBuilder 会在配置文件中读取 对应的 Interceptor:
private void pluginElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
//解析Interceptor
String interceptor = child.getStringAttribute("interceptor");
Properties properties = child.getChildrenAsProperties();
Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).getDeclaredConstructor().newInstance();
interceptorInstance.setProperties(properties);
configuration.addInterceptor(interceptorInstance);
}
}
}
- 最终所有配置好的 interceptor 会被添加到 InterceptorChain 的 List 集合中;
在创建这4个核心对象后,InterceptorChain 都会尝试执行 pluginAll 方法 是否需要创建代理对象增强该类:
//创建ParameterHandler
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
return parameterHandler;
}
// 创建ResultSetHandler
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;
}
//创建StatementHandler
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;
}
//创建Executor
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;
}
在 InterceptorChain 的pluginAll 方法,中会调用 Interceptor 接口的默认方法 plugin 方法进行判断:
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}
我们再看看 plugin 方法,实则是调用 Plugin 的 wrap 方法:
default Object plugin(Object target) {
return Plugin.wrap(target, this);
}
Plugin 的 wrap 方法如下:
public static Object wrap(Object target, Interceptor interceptor) {
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
if (interfaces.length > 0) {
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
}
return target;
}
- 先获取 Interceptor 注解上的参数,比如 class 类,方法名称,方法参数,这三个参数可以准确获取某个类的某个方法;
- 接着判断当前的 Interceptor 是否符合当前类,如果符合就去创建代理对象;
- signatureMap 的 key 是类的对象,value 是该类对象中的方法 set 集合;
在 Plugin 的 Invoke 方法中:
@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)) {
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
- 先判断当前方法是不是存在于 Set 集合中,如果存在就执行 interceptor 的 intercept 方法,如果不存在就执行原方法;
2.3 PageHelper 自定义 Mybatis 拦截器类
PageHelper 自定义的拦截器 PageInterceptor 类中,会调用 ExecutorUtil 类的 pageQuery 方法去执行分页操作
@Intercepts(
{
@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 {
@Override
public Object intercept(Invocation invocation) throws Throwable {
try {
//...
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, null, 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);
}
}
// 进行分页处理
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 {
if(dialect != null){
dialect.afterAll();
}
}
}
}
- 可见,它是针对 Executor 的 query 方法进行增强,目的是获取 BoundSql 类中的原 SQL。
ExecutorUtil.pageQuery 里面会调用抽象类 AbstractHelperDialect 类的 getPageSql 方法来实现对 BoundSql 中 sql 改写:
public String getPageSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {
String sql = boundSql.getSql();
//从ThreadLoacl中拿出分页参数
Page page = getLocalPage();
String orderBy = page.getOrderBy();
if (StringUtil.isNotEmpty(orderBy)) {
pageKey.update(orderBy);
sql = OrderByParser.converToOrderBySql(sql, orderBy);
}
if (page.isOrderByOnly()) {
return sql;
}
//调用子类的getPageSql方法
return getPageSql(sql, page, pageKey);
}
但实际上进行改写的是实现类 MySqlDialect:
@Override
public String getPageSql(String sql, Page page, CacheKey pageKey) {
StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);
sqlBuilder.append(sql);
if (page.getStartRow() == 0) {
sqlBuilder.append("\n LIMIT ? ");
} else {
sqlBuilder.append("\n LIMIT ?, ? ");
}
return sqlBuilder.toString();
}
- 可见上面的代码就是在原 SQL 中拼接分页参数