一、简介
本文介绍Spring中的事务的传播机制,并且用实例说明它的用法。
本内容也是Java后端面试常见的问题。
二、概述
Spring在TransactionDefinition接口中规定了7种类型的事务传播行为。Propagation枚举则引用了这些类型,开发过程中我们一般直接用Propagation枚举。常用的三项已经加粗。
本处所测试的皆是运行addEmpByRequired方法。
三、七中事务传播机制
3.1、Propagation.REQUIRED(默认是这个)
员工service
@Service
public class EmployeeServiceImpl implements EmployeeService {
@Autowired
EmployeeMapper employeeMapper;
@Autowired
DepartmentService departmentService;
/**
* 添加员工的同时添加部门
* REQUIRED 传播
* @param name 员工姓名
*/
@Override
@Transactional(propagation = Propagation.REQUIRED)
public void addEmpByRequired(String name) {
Employee employee = new Employee();
employee.setDeptId(1);
employee.setName(name);
employee.setAddress("邯郸");
employeeMapper.insertSelective(employee);
departmentService.addDept("jishubu");
int i = 1/0;
}
}
部门service
@Service
public class DepartmentServiceImpl implements DepartmentService {
@Autowired
DepartmentMapper departmentMapper;
@Override
public void addDeptByRequired(String name) {
Department department = new Department();
department.setName(name);
departmentMapper.insertSelective(department);
// int i = 1/0;
}
}
- 上述代码中,无论int i =1/0 这个异常出现在哪里,添加员工和添加部门都会回滚。因为 REQUIRED 会让添加员工和添加部门变为一个事务。
- 如果在addDeptByRequired上添加@Transactional(propagation = Propagation.REQUIRED),在addEmpByRequired不添加事务,则addDeptByRequired是一个事务,addEmpByRequired并不是一个事务。因为addDeptByRequired开启了一个事务,但是addEmpByRequired并不存在一个事务中。
3.2、Propagation.SUPPORTS
员工service
@Override
@Transactional(propagation = Propagation.REQUIRED)
public void addEmpBySupports(String name) {
Employee employee = new Employee();
employee.setDeptId(2);
employee.setName(name);
employee.setAddress("邯郸");
employeeMapper.insertSelective(employee);
departmentService.addDeptBySupports("jishubu");
// int i = 1/0;
}
部门service
@Override
@Transactional(propagation = Propagation.SUPPORTS)
public void addDeptBySupports(String name) {
Department department = new Department();
department.setName(name);
departmentMapper.insertSelective(department);
int i = 1/0;
}
- 关于这个属性,在以上代码中,主要是添加到addDeptBySupports上的,也就是被调用方法上。因为添加到addEmpBySupports就不以事务的方式运行了。
- 然后,如果addEmpBySupports为事务,则addDeptBySupports也为事务。如果addEmpBySupports不是事务,则addDeptBySupports也不是事务。
3.3、Propagation.MANDATORY
员工service
@Override
// @Transactional(propagation = Propagation.REQUIRED)
public void addEmpByMandatory(String name) {
System.out.println("aaaaaa");
Employee employee = new Employee();
employee.setDeptId(3);
employee.setName(name);
employee.setAddress("邯郸");
employeeMapper.insertSelective(employee);
departmentService.addDeptByMandatory("jishubu");
int i = 1/0;
}
部门service
@Override
@Transactional(propagation = Propagation.MANDATORY)
public void addDeptByMandatory(String name) {
Department department = new Department();
department.setName(name);
departmentMapper.insertSelective(department);
int i = 1/0;
}
- 这个属性也是添加到addDeptByMandatory(被调用者) 上的。若添加到addEmpByMandatory(调用者)上,则直接抛出异常。
- 该属性添加到addDeptByMandatory上, 如果addEmpByMandatory有事务,则addDeptByMandatory加入到addEmpByMandatory的事务中,如果addEmpByMandatory没有事务,则直接抛出异常。
3.4、Propagation.REQUIRES_NEW
员工service
@Override
@Transactional(propagation = Propagation.REQUIRES_NEW)
public void addEmpByRequiresNew(String name) {
Employee employee = new Employee();
employee.setDeptId(4);
employee.setName(name);
employee.setAddress("邯郸");
employeeMapper.insertSelective(employee);
departmentService.addDeptByRequiresNew("jishubu");
int i = 1/0;
}
部门service
@Override
@Transactional(propagation = Propagation.REQUIRES_NEW)
public void addDeptByRequiresNew(String name) {
Department department = new Department();
department.setName(name);
departmentMapper.insertSelective(department);
// int i = 1/0;
}
- 这个属性应该是除了REQUIRED用的最多的。这个属性也是针对被调用者的(addDeptByRequiresNew)。
- 不管调用者(addEmpByRequiresNew)是否存在事务,被调用者(addDeptByRequiresNew)都会新开一个事务,相当于被调用者都存在于自己的事务中和调用者没有关系。
- 如上述代码,addEmpByRequiresNew会回滚,但addDeptByRequiresNew不会回滚。因为他们是两个事务。
3.5、 Propagation.NOT_SUPPORTED
员工service
@Override
@Transactional(propagation = Propagation.REQUIRED)
public void addEmpByNotSupported(String name) {
Employee employee = new Employee();
employee.setDeptId(5);
employee.setName(name);
employee.setAddress("邯郸");
employeeMapper.insertSelective(employee);
departmentService.addDeptByNotSupported("jishubu");
int i = 1/0;
}
部门service
@Override
@Transactional(propagation = Propagation.NOT_SUPPORTED)
public void addDeptByNotSupported(String name) {
Department department = new Department();
department.setName(name);
departmentMapper.insertSelective(department);
int i = 1/0;
}
- 这个属性如果放在调用者(addEmpByNotSupported)上,则是以非事务方式运行。
- 如果放在被调用者(addDeptByNotSupported)上,该方法(addDeptByNotSupported)以非事务运行,调用者如果有事务,则运行单独的事务(挂起)。
- 上述代码,会出现添加员工回滚,添加部门不回滚。
3.6、Propagation.NEVER
员工service
@Override
@Transactional(propagation = Propagation.REQUIRED)
public void addEmpByNever(String name) {
Employee employee = new Employee();
employee.setDeptId(6);
employee.setName(name);
employee.setAddress("邯郸");
employeeMapper.insertSelective(employee);
departmentService.addDeptByNever("jishubu");
int i = 1/0;
}
部门service
@Override
@Transactional(propagation = Propagation.NEVER)
public void addDeptByNever(String name) {
Department department = new Department();
department.setName(name);
departmentMapper.insertSelective(department);
int i = 1/0;
}
- 这个属性如果在调用者上,则直接以非事务运行。如果作用在被调用者上,则看调用者是否有事务,如果调用者有事务,则抛出异常,如果没有事务,则以非事务运行。
- 上述代码中,则会抛出异常。(并不是除0异常,而是:Existing transaction found for transaction marked with propagation 'never')
3.7、Propagation.NESTED
说明
PROPAGATION_NESTED 启动的事务内嵌于外部事务中(如果存在外部事务的话)。嵌套子事务是JDBC 的保存点(SavePoint)的一个应用,它是具有可回滚到多个保存点的单个物理事务。被调用者使用PROPAGATION_NESTED时,需JDBC 3.0及以上和JDK1.4及以上,且实现者需要支持保存点事务机制。
提交
只有通过外部的事务提交,才能引起内部事务的提交。(内嵌事务不是一个独立的事务,它依赖于外部事务)
回滚
父事务回滚,子事务一定回滚
子事务回滚到savepoint,父事务可选择性回滚或者不不滚
员工service
@Override
@Transactional(propagation = Propagation.REQUIRED) //1
public void addEmpByNested(String name) {
Employee employee = new Employee();
employee.setDeptId(7);
employeeMapper.insertSelective(employee); //2
try {
departmentService.addDeptByNested("jishubu"); //3
}catch (Exception e){
// 其他业务, 如 departmentService.methodC(); //5
}
employee.setDeptId(8);
employeeMapper.update(employee); //6
// int i = 1/0;
}
部门service
@Override
@Transactional(propagation = Propagation.NESTED)
public void addDeptByNested(String name) {
Department department = new Department();
department.setName(name);
departmentMapper.insertSelective(department); //4
// int i = 1/0;
}
1. 将开起新事务A
2. 插入数据,事务A挂起,没有commit,
3. 新建一个savepoint保存2插入的数据,不提交。
4. 若addDeptByNested异常,回滚4的操作,不影响2,同时可处理后面的5,6逻辑,最后一起commit: 2,5,6, 若addDeptByNested无异常,最后一起commit: 2,4,6
假如methodB使用的PROPAGATION_REQUIRES_NEW,那么B异常,会commit: 2,5,6,和NESTED一致。
如果methodB没有出现异常,那么会先commit4,再commit:6,那么事务将分离开,不能保持一致,假如执行6报错,2和6将回滚,而4却没有被回滚,不能达到预期效果。
并不是只有NESTED用于嵌套,只要理解上述7个传播机制的意思,都可以嵌套用。