PageHelper在SpringBoot中的使用和原理分析
在SpringBoot项目中使用Mybatis的PageHelper分页插件进行分页查询
1、导入相关依赖
<dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><scope>runtime</scope>
</dependency>
<dependency><groupId>org.mybatis.spring.boot</groupId><artifactId>mybatis-spring-boot-starter</artifactId><version>2.3.2</version>
</dependency>
<dependency><groupId>com.github.pagehelper</groupId><artifactId>pagehelper-spring-boot-starter</artifactId><version>2.1.0</version>
</dependency>
2、添加相关配置信息
spring:datasource:driver-class-name: com.mysql.cj.jdbc.Driverusername: rootpassword: rooturl: jdbc:mysql://localhost:3306/ssm?useUnicode=true&characterEncoding=UTF-8&serverTimezone=Asia/Shanghai&nullCatalogMeansCurrent=truepagehelper:reasonable: truedefaultCount: true # 分页插件默认参数支持 default-count 形式，自定义扩展的参数，必须大小写一致helperDialect: mysql
mybatis:mapper-locations: classpath:/mapper/*.xml
3、编写mapper接口
@Repository
public interface UserMapper {List<User> getAllUser();
}
4、编写service，其中getAllUser()是普通查询，getAllUser1()和getAllUser2()是分页查询，使用PageHelper需要传入pageNum和pageSize
@Service
public class UserService {@Autowiredprivate UserMapper userMapper;public List<User> getAllUser(){return userMapper.getAllUser();}public List<User> getAllUser1(int pageNum,int pageSize){PageHelper.startPage(pageNum,pageSize);List<User> allUser = userMapper.getAllUser();return allUser;}public PageInfo<User> getAllUser2(int pageNum,int pageSize){PageHelper.startPage(pageNum,pageSize);List<User> allUser = userMapper.getAllUser();PageInfo<User> pageInfo = new PageInfo<>();pageInfo.setList(allUser);return pageInfo;}
}
5、编写Controller
@RestController
public class UserController {@Autowiredprivate UserService userService;@GetMapping("/users")public List<User> getAllUser(){return userService.getAllUser();}@GetMapping("/users1")public List<User> getAllUser1(){return userService.getAllUser1(1,3);}@GetMapping("/users2")public PageInfo<User> getAllUser2(){return userService.getAllUser2(1,3);}
}
6、编写Mybatis的mapper文件，其中namespace要与接口方法所在的包绑定，MappedStatement的id要与接口方法绑定
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.demo.mapper.UserMapper"><!-- 通用查询映射结果 --><resultMap id="BaseResultMap" type="com.example.demo.pojo.User"><id column="id" property="id" /><result column="name" property="name" /><result column="phone" property="phone" /></resultMap><select id="getAllUser" resultType="com.example.demo.pojo.User">select * from user</select>
</mapper>
7、测试程序，测试接口
接口：http://localhost:8080/users
[{"id":"1","name":"zlw","phone":"17860397215"},{"id":"2","name":"zyx","phone":"18865986031"},{"id":"3","name":"zq","phone":"17860397216"},{"id":"4","name":"zxj","phone":"17860398476"},{"id":"5","name":"zmj","phone":"15021469872"},{"id":"6","name":"zqq","phone":"17652369421"}]
接口：http://localhost:8080/users1
[{"id":"1","name":"zlw","phone":"17860397215"},{"id":"2","name":"zyx","phone":"18865986031"},{"id":"3","name":"zq","phone":"17860397216"}]
接口：http://localhost:8080/users2
{"total":0,"list":[{"id":"1","name":"zlw","phone":"17860397215"},{"id":"2","name":"zyx","phone":"18865986031"},{"id":"3","name":"zq","phone":"17860397216"}],"pageNum":0,"pageSize":0,"size":0,"startRow":0,"endRow":0,"pages":0,"prePage":0,"nextPage":0,"isFirstPage":false,"isLastPage":false,"hasPreviousPage":false,"hasNextPage":false,"navigatePages":0,"navigatepageNums":null,"navigateFirstPage":0,"navigateLastPage":0}
接下来从代码的角度看看PageHelper是怎么实现分页的。
当导入pagehelper-spring-boot-starter依赖后，pagehelper已经自定了一个自动配置类PageHelperAutoConfiguration，可以在maven包下找到对应的spring.factories文件看一下。
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.github.pagehelper.autoconfigure.PageHelperAutoConfiguration
进入这个类看一下
@Configuration
@ConditionalOnBean({SqlSessionFactory.class})
@EnableConfigurationProperties({PageHelperProperties.class, PageHelperStandardProperties.class})
@AutoConfigureAfter({MybatisAutoConfiguration.class})
@Lazy(false)
public class PageHelperAutoConfiguration implements InitializingBean {private final List<SqlSessionFactory> sqlSessionFactoryList;private final PageHelperProperties properties;public PageHelperAutoConfiguration(List<SqlSessionFactory> sqlSessionFactoryList, PageHelperStandardProperties standardProperties) {this.sqlSessionFactoryList = sqlSessionFactoryList;this.properties = standardProperties.getProperties();}public void afterPropertiesSet() throws Exception {PageInterceptor interceptor = new PageInterceptor();interceptor.setProperties(this.properties);Iterator var2 = this.sqlSessionFactoryList.iterator();while(var2.hasNext()) {SqlSessionFactory sqlSessionFactory = (SqlSessionFactory)var2.next();org.apache.ibatis.session.Configuration configuration = sqlSessionFactory.getConfiguration();if (!this.containsInterceptor(configuration, interceptor)) {configuration.addInterceptor(interceptor);}}}private boolean containsInterceptor(org.apache.ibatis.session.Configuration configuration, Interceptor interceptor) {try {return configuration.getInterceptors().stream().anyMatch((config) -> {return interceptor.getClass().isAssignableFrom(config.getClass());});} catch (Exception var4) {return false;}}
}
可以看到这个自动配置类PageHelperAutoConfiguration实现了InitializingBean接口并且重写了afterPropertiesSet()方法，所以当spring在初始化PageHelperAutoConfiguration时会执行afterPropertiesSet()进行初始化。这里创建了一个PageInterceptor分页拦截器，然后将PageInterceptor配置到Mybatis中。
这个PageInterceptor是实现分页的关键，点进去看看，先看一下类上的注解。
@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}
)})
@Intercepts表明PageInterceptor是一个拦截器，@Signature定义了这了拦截器要拦截的类型，method = "query"也就是这个拦截器会拦截所有的sql查询方法。
回到UserService.java，在调用mapper查询的代码上打个断点看一下PageInterceptor是怎么执行的。
@Service
public class UserService {@Autowiredprivate UserMapper userMapper;public List<User> getAllUser(){return userMapper.getAllUser();}public List<User> getAllUser1(int pageNum,int pageSize){PageHelper.startPage(pageNum, pageSize);List<User> allUser = userMapper.getAllUser();return allUser;}public PageInfo<User> getAllUser2(int pageNum,int pageSize){PageHelper.startPage(pageNum,pageSize);List<User> allUser = userMapper.getAllUser();PageInfo<User> pageInfo = new PageInfo<>();pageInfo.setList(allUser);return pageInfo;}
}
程序首先会执行PageInterceptor的plugin()方法，它返回了Mybatis执行器的代理对象，跟进wrap()方法看看。
public class PageInterceptor implements Interceptor {//...public Object plugin(Object target) {return Plugin.wrap(target, this);}//...
}
public class Plugin implements InvocationHandler {private final Object target;private final Interceptor interceptor;private final Map<Class<?>, Set<Method>> signatureMap;private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {this.target = target;this.interceptor = interceptor;this.signatureMap = signatureMap;}public static Object wrap(Object target, Interceptor interceptor) {//map的key是Mybatis的执行器，value是要执行拦截的查询方法Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);//target是一个cachingExcutor类，这是Mybatis的执行器，负责执行查询操作和缓存机制Class<?> type = target.getClass();Class<?>[] interfaces = getAllInterfaces(type, signatureMap);//为cachingExcutor生成一个代理对象，看第三个参数，Plugin类自身实现了Invocation接口，这里把自身传进去了return interfaces.length > 0 ? Proxy.newProxyInstance(type.getClassLoader(), interfaces, new Plugin(target, interceptor, signatureMap)) : target;}//Plugin类实现Invocation接口，并重写invoke方法public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {Set<Method> methods = (Set)this.signatureMap.get(method.getDeclaringClass());//如果方法需要拦截，就执行PageIntercptor的intercept()方法return methods != null && methods.contains(method) ? this.interceptor.intercept(new Invocation(this.target, method, args)) : method.invoke(this.target, args);} catch (Exception var5) {throw ExceptionUtil.unwrapThrowable(var5);}}
}
Plugin的wrap()方法最终返回了一个Mybatis执行器的代理对象，在执行分页查询方法时，通过invoke方法进行调用，这时会执行PageInterceptor的intercept方法。
//PageInterceptor.class
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;if (args.length == 4) {boundSql = ms.getBoundSql(parameter);cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);} else {cacheKey = (CacheKey)args[4];boundSql = (BoundSql)args[5];}this.checkDialectExists();if (this.dialect instanceof BoundSqlInterceptor.Chain) {boundSql = ((BoundSqlInterceptor.Chain)this.dialect).doBoundSql(Type.ORIGINAL, boundSql, cacheKey);}List resultList;if (!this.dialect.skip(ms, parameter, rowBounds)) {this.debugStackTraceLog();Future<Long> countFuture = null;Long count;if (this.dialect.beforeCount(ms, parameter, rowBounds)) {if (this.dialect.isAsyncCount()) {countFuture = this.asyncCount(ms, boundSql, parameter, rowBounds);} else {count = this.count(executor, ms, parameter, rowBounds, (ResultHandler)null, boundSql);if (!this.dialect.afterCount(count, parameter, rowBounds)) {Object var13 = this.dialect.afterPage(new ArrayList(), parameter, rowBounds);return var13;}}}//分页的重点resultList = ExecutorUtil.pageQuery(this.dialect, executor, ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);if (countFuture != null) {count = (Long)countFuture.get();this.dialect.afterCount(count, parameter, rowBounds);}} else {resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);}Object var17 = this.dialect.afterPage(resultList, parameter, rowBounds);return var17;} finally {if (this.dialect != null) {this.dialect.afterAll();}}
}
ExecutorUtil.pageQuery()方法是分页逻辑的重点部分，进去看看。
//ExecutorUtil.class
public static <E> List<E> pageQuery(Dialect dialect, Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql, CacheKey cacheKey) throws SQLException {if (!dialect.beforePage(ms, parameter, rowBounds)) {return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);} else {parameter = dialect.processParameterObject(ms, parameter, boundSql, cacheKey);//从mysql方言中获取分页sql，在原始sql上拼接limitString pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, cacheKey);//创建一个BoundSql对象，包含了分页sql以及其他的参数BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);Map<String, Object> additionalParameters = getAdditionalParameter(boundSql);Iterator var12 = additionalParameters.keySet().iterator();while(var12.hasNext()) {String key = (String)var12.next();pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));}if (dialect instanceof BoundSqlInterceptor.Chain) {pageBoundSql = ((BoundSqlInterceptor.Chain)dialect).doBoundSql(Type.PAGE_SQL, pageBoundSql, cacheKey);}//执行器去查询return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, pageBoundSql);}
}
从dialect.getPageSql()方法一直跟进可以进行mysql方言的getPageSql()方法，可以看到这个方法在原始的sql上拼接上了limit语句
//MySqlDialect.class
public String getPageSql(String sql, Page page, CacheKey pageKey) {StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);sqlBuilder.append(sql);if (page.getStartRow() == 0L) {sqlBuilder.append("\n LIMIT ? ");} else {sqlBuilder.append("\n LIMIT ?, ? ");}return sqlBuilder.toString();
}
接下来会封装一个BoundSql对象，包含分页sql和一些参数。
取哪些数据是根据pageNum和pageSize来计算的，这些参数是在调用mapper接口的方法之前通过PageHelper的startPage方法传入的。
PageHelper.startPage(pageNum, pageSize);
最后调用执行器的query()方法从数据库中查询结果并将其返回。
