SpringBoot笔记6---如何读写数据库之使用JpaSpecificationExecutor读写数据库

关于如何配置数据库以及需要添加哪些依赖包，在《SpringBoot笔记3---如何读写数据库之使用CrudRepository中提供的方法来读写数据库》中有详细说明，这里不再赘述。本文基于SpringBoot笔记3一文的基础上，说明如何通过JpaSpecificationExecutor来读写数据库。

注意：JpaSpecificationExecutor一个重要的特点是可以实现分页读取数据库。

1 新建UserPersistence类如下：

package com.gm.springboot_test.persistence;

import java.util.List;

import com.gm.springboot_test.entity.User;

public interface UserPersistence {
public List<User> getByName(String name);
public List<Integer> getAgeByName(String name);
}


2 新建UserDao类如下：
package com.gm.springboot_test.dao;

import org.springframework.data.jpa.repository.JpaSpecificationExecutor;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;

import com.gm.springboot_test.entity.User;

@Repository
public interface UserDao extends CrudRepository<User, Integer>,
JpaSpecificationExecutor<User> {

}注意：
一定要继承CrudRepository和JpaSpecification类。

3 新建UserPersistenceImpl类如下：

package com.gm.springboot_test.persistence.impl;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Predicate;
import javax.persistence.criteria.Root;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.domain.Page;
import org.springframework.data.domain.PageRequest;
import org.springframework.data.domain.Pageable;
import org.springframework.data.domain.Sort;
import org.springframework.data.jpa.domain.Specification;
import org.springframework.stereotype.Repository;

import com.gm.springboot_test.dao.UserDao;
import com.gm.springboot_test.entity.User;
import com.gm.springboot_test.persistence.UserPersistence;

@Repository
public class UserPersistenceImpl implements UserPersistence {

@Autowired
private UserDao userDao;

@Override
public List<User> getByName(final String name) {
// TODO Auto-generated method stub
int pageIndex = 0;
int pageSize = 5;

Pageable pageable = new PageRequest(pageIndex, pageSize, new Sort("age"));

Page<User> ens = userDao.findAll(new Specification<User>() {

@Override
public Predicate toPredicate(Root<User> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
// TODO Auto-generated method stub
Predicate predicate = cb.equal(root.get("name").as(String.class), name);
return predicate;
}

}, pageable);

List<User> userList = ens.getContent();
return userList;
}

@Override
public List<Integer> getAgeByName(final String name) {
// TODO Auto-generated method stub
List<User> userList = userDao.findAll(new Specification<User>() {

@Override
public Predicate toPredicate(Root<User> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
// TODO Auto-generated method stub
Predicate predicate = cb.equal(root.get("name").as(String.class), name);
return predicate;
}

});

List<Integer> ageList = new ArrayList();
for (User user : userList) {
ageList.add(user.getAge()); // age index is 2
}

return ageList;
}
}