SpringBoot自动装配&事务传播策略

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}
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为何使用SpringBoot后，我们只需上述几行代码即可搭建一个web服务器，比之前使用SpringMVC不要简洁太多。

这其中奥妙在于 @SpringBootApplication 注解之中：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM,
				classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
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其又继承了 @EnableAutoConfiguration 注解：

/** 
 * ...
 * Auto-configuration classes are regular Spring {@link Configuration} beans. They are
 * located using the {@link SpringFactoriesLoader} ..
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
	...
}
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查看注释可知，自动配置类也是以常规的Spring Bean的形式存在。它们被 SpringFactoriesLoader 定位：

public final class SpringFactoriesLoader {
    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if (result != null) {
            return result;
        } else {
            try {
                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
                LinkedMultiValueMap result = new LinkedMultiValueMap();

                ...
            } catch (IOException var13) {
                throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
            }
        }
    }
}
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该类会从类路径中的 "META-INF/spring.factories" 中读取自动装配的类列表，其中 spring-boot-autoconfigure.jar 中的就包含了内嵌Tomcat、SpringMVC、事务等功能的配置类：

org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration
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事务传播策略

前言

方法为维度

由于Spring事务是基于AOP的，所以事务以方法为维度存在于Java代码中。而事务的传播是基于多事务之间相互影响的，所以在代码中表现为一个事务方法调用另一个事务方法（如下列代码中 savePersons 方法中调用 saveChildren ）：

@Service
public class PersonService {

    @Autowired
    private PersonMapper personMapper;
    
    @Transactional
    public void savePersons() {
        Person person = new Person();
        person.setUsername("parent");
        person.setPassword("123");
        personMapper.insertSelective(person);
        
        saveChildren();
    }

    @Transactional
    public void saveChildren() {
        saveChild1();
        saveChild2();
        int i = 1 / 0;
    }

    public void saveChild1() {
        Person person = new Person();
        person.setUsername("child1");
        person.setPassword("456");
        personMapper.insertSelective(person);
    }

    public void saveChild2() {
        Person person = new Person();
        person.setUsername("child2");
        person.setPassword("789");
        personMapper.insertSelective(person);
    }

}
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bean为入口

但是，SpringAOP是基于bean增强的，也就是说当你调用一个bean的事务方法（被事务注解修饰的方法）时，该事务注解是可以正常生效的。但如果你调用本类中的事务方法，那就相当于将该方法中的代码内嵌到当前方法中，即该方法的事务注解会被忽略。

例如：

@Service
public class PersonService {

    @Autowired
    private PersonMapper personMapper;

    public void savePersons() {
        Person person = new Person();
        person.setUsername("parent");
        person.setPassword("123");
        personMapper.insertSelective(person);
        
        saveChildren();
    }

    @Transactional
    public void saveChildren() {
        saveChild1();
        saveChild2();
        int i = 1 / 0;
    }
}
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上列代码等效于下列代码（ saveChildren 方法事务注解被忽略掉了）：

@Service
public class PersonService {

    @Autowired
    private PersonMapper personMapper;

    public void savePersons() {
        Person person = new Person();
        person.setUsername("parent");
        person.setPassword("123");
        personMapper.insertSelective(person);
        
        saveChild1();
        saveChild2();
        int i = 1 / 0;
    }
}
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因此我们接下来讨论的一个事务/非事务方法调用另一个事务/非事务方法，这两个方法被调用的方式都是基于bean作为方法引用的，而非通过 this 调用本类中的方法。因此我们不妨将两个写库方法 saveChildren 和 savePersons 移入两个bean中进行测试：

@Service
public class PersonService2 {
    @Autowired
    private PersonMapper personMapper;

    public void saveChildren() {
        saveChild1();
        saveChild2();
    }

    public void saveChild1() {
        Person person = new Person();
        person.setUsername("child1");
        person.setPassword("456");
        personMapper.insertSelective(person);
    }

    public void saveChild2() {
        Person person = new Person();
        person.setUsername("child2");
        person.setPassword("789");
        personMapper.insertSelective(person);
    }
}
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@Service
public class PersonService {

    @Autowired
    private PersonMapper personMapper;

    @Autowired
    private PersonService2 personService2;

    public void savePersons() {
        Person person = new Person();
        person.setUsername("parent");
        person.setPassword("123");
        personMapper.insertSelective(person);

        personService2.saveChildren();
    }	
}
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@RunWith(SpringRunner.class)
@SpringBootTest(classes = Application.class)
public class TransTest {

    @Autowired
    private PersonService personService;

    @Test
    public void test() {
        personService.savePersons();
    }
}
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当前事务&父方法事务

本文中说父方法是否创建事务，不仅仅是指调用本方法的调用方，而是泛指方法调用链的上游方法，只要上游方法中的任意一个方法开启了事务，那么当前方法的执行就处于事务之中，也即执行当前方法时存在事务。

策略枚举

Spring事务传播策略相关的枚举类如下：

package org.springframework.transaction.annotation;

public enum Propagation {
    REQUIRED(0),
    SUPPORTS(1),
    MANDATORY(2),
    REQUIRES_NEW(3),
    NOT_SUPPORTED(4),
    NEVER(5),
    NESTED(6);

    private final int value;

    private Propagation(int value) {
        this.value = value;
    }

    public int value() {
        return this.value;
    }
}
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REQUIRED——有饭就吃，没饭自己买

@Transactional 注解默认的传播策略就是 REQUIRED ：

public @interface Transactional {
    Propagation propagation() default Propagation.REQUIRED;
}
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/**
	 * Support a current transaction, create a new one if none exists.
	 * Analogous to EJB transaction attribute of the same name.
	 * <p>This is the default setting of a transaction annotation.
	 */
REQUIRED(TransactionDefinition.PROPAGATION_REQUIRED),
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被该注解表示：如果当前方法有事务，则支持当前事务；如果当前方法没有事务，则新建事务供自己使用。

父无，子自力更生

public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
}
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@Transactional(propagation = Propagation.REQUIRED)
public void saveChildren() {
    saveChild1();
    saveChild2();
    int i = 1/0;
}
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由于父方法没有注解，执行到第7行时，调用了传播策略为REQUIRED的事务方法，其自己新建事务供自己使用，因此 child1, child2 因为 1/0 异常不会被插入，异常抛至父方法，父方法因为没有事务所以不会回滚之前插入的 parent ，执行结果如下：

----+----------+----------+
| id | username | password |
+----+----------+----------+
| 23 | parent   | 123      |
+----+----------+----------+

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父有，子继承

@Transactional(propagation = Propagation.REQUIRED)
public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
}

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@Transactional(propagation = Propagation.REQUIRED)
public void saveChildren() {
    saveChild1();
    saveChild2();
    int i = 1 / 0;
}

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@Autowired
private PersonService personService;

@Test
public void test() {
    personService.savePersons();
}

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由于 test 调用bean personService的事务方法savePersons且其传播策略为REQUIRED，于是其新建一个事务给自己用，当调用bean personService2的REQUIRED事务方法时，发现当前有事务因此支持当前事务，因此parent、child1、child2的插入由于在同一个事务中，因此在1/0异常抛出后都被回滚：

mysql> select * from person;
Empty set (0.00 sec)

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俚语

REQUIRED，老板（父方法）有饭吃（有事务），我（子方法）跟着老板吃（支持当前事务）；老板没饭吃，我自己买饭吃

SUPPORTS——有饭就吃，没饭饿肚子

/**
	 * Support a current transaction, execute non-transactionally if none exists.
	 * Analogous to EJB transaction attribute of the same name.
	 * <p>Note: For transaction managers with transaction synchronization,
	 * {@code SUPPORTS} is slightly different from no transaction at all,
	 * as it defines a transaction scope that synchronization will apply for.
	 * As a consequence, the same resources (JDBC Connection, Hibernate Session, etc)
	 * will be shared for the entire specified scope. Note that this depends on
	 * the actual synchronization configuration of the transaction manager.
	 * @see org.springframework.transaction.support.AbstractPlatformTransactionManager#setTransactionSynchronization
	 */
	SUPPORTS(TransactionDefinition.PROPAGATION_SUPPORTS),

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如果当前有事务，则支持当前事务，否则以非事务的方式执行当前方法

父有子支持

当父方法会创建事务时，子方法用SUPPORTS和用REQUIRED效果是一样的

@Transactional(propagation = Propagation.REQUIRED)
public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
}

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@Transactional(propagation = Propagation.SUPPORTS)
public void saveChildren() {
    saveChild1();
    saveChild2();
    int i = 1 / 0;
}

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mysql> select * from person;
Empty set (0.00 sec)

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父无子无

当父方法没有创建事务，那么子方法也不会自作主张去新建事务：

public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
}

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@Transactional(propagation = Propagation.SUPPORTS)
public void saveChildren() {
    saveChild1();
    saveChild2();
    int i = 1 / 0;
}

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mysql> select * from person;
+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 29 | parent   | 123      |
| 30 | child1   | 456      |
| 31 | child2   | 789      |
+----+----------+----------+

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俚语

SUPPORTS：老板有饭吃，我跟着老板吃；老板没饭吃，我就只能饿肚子了。

MANDATORY——必须要有饭吃

/**
	 * Support a current transaction, throw an exception if none exists.
	 * Analogous to EJB transaction attribute of the same name.
	 */
	MANDATORY(TransactionDefinition.PROPAGATION_MANDATORY),

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强制以事务的方式执行当前方法：如果当前有事务，那么支持当前事务，否则抛出异常

父有子支持

这点REQUIRED, SUPPORTS, MANDATORY是一样的

父无子罢工

public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
}

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@Transactional(propagation = Propagation.MANDATORY)
public void saveChildren() {
    saveChild1();
    saveChild2();
    int i = 1 / 0;
}

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当执行到personService2的MANDATORY事务方法时，发现当前没有事务，于是它直接抛出一个异常：

org.springframework.transaction.IllegalTransactionStateException: No existing transaction found for transaction marked with propagation 'mandatory'

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mysql> select * from person;
+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 32 | parent   | 123      |
+----+----------+----------+

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俚语

MANDATORY：老板有饭吃，跟着老板吃；老板没饭吃，老子不干了。有饭才干活

REQUIRES_NEW——自力更生

/**
	 * Create a new transaction, and suspend the current transaction if one exists.
	 * Analogous to the EJB transaction attribute of the same name.
	 * <p><b>NOTE:</b> Actual transaction suspension will not work out-of-the-box
	 * on all transaction managers. This in particular applies to
	 * {@link org.springframework.transaction.jta.JtaTransactionManager},
	 * which requires the {@code javax.transaction.TransactionManager} to be
	 * made available to it (which is server-specific in standard Java EE).
	 * @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
	 */
	REQUIRES_NEW(TransactionDefinition.PROPAGATION_REQUIRES_NEW),

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不管当前有没有事务，自己都会新建一个事务为自己所用，并且如果当前有事务那么就会挂起当前事务

父无子自强

public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
}

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@Transactional(propagation = Propagation.REQUIRES_NEW)
public void saveChildren() {
    saveChild1();
    saveChild2();
    int i = 1 / 0;
}

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此情景下，REQUIRED_NEW同REQUIRED

mysql> select * from person;
+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 33 | parent   | 123      |
+----+----------+----------+

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父有子也不稀罕

@Transactional(propagation = Propagation.REQUIRED)
public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
    
    int i = 1 / 0;
}

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@Transactional(propagation = Propagation.REQUIRES_NEW)
public void saveChildren() {
    saveChild1();
    saveChild2();
}

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这次我将 1/0 移入了父方法中，父方法有事务因此parent的插入会回滚，但是子方法的执行会挂起当前事务另建新事务，因此子方法的插入依然有效（子方法执行结束后父方法的事务又会被自动恢复）

mysql> select * from person;
+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 40 | child1   | 456      |
| 41 | child2   | 789      |
+----+----------+----------+

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俚语

REQUIRED_NEW：不管老板有没有饭吃，我都自己买饭吃，不接受他人的恩惠。

NOT_SUPPORTED——不吃饭只干活

/**
	 * Execute non-transactionally, suspend the current transaction if one exists.
	 * Analogous to EJB transaction attribute of the same name.
	 * <p><b>NOTE:</b> Actual transaction suspension will not work out-of-the-box
	 * on all transaction managers. This in particular applies to
	 * {@link org.springframework.transaction.jta.JtaTransactionManager},
	 * which requires the {@code javax.transaction.TransactionManager} to be
	 * made available to it (which is server-specific in standard Java EE).
	 * @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
	 */
	NOT_SUPPORTED(TransactionDefinition.PROPAGATION_NOT_SUPPORTED),

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强制以非事务的方式执行当前代码，如果当前有事务则将其挂起。

父有子不用

@Transactional(propagation = Propagation.REQUIRED)
public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
}

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@Transactional(propagation = Propagation.NOT_SUPPORTED)
public void saveChildren() {
    saveChild1();
    int i = 1 / 0;
    saveChild2();
}

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执行子方法时，事务被挂起，因此child1的插入未被回滚，回到父方法后事务被恢复，因此parent的插入被回滚

+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 43 | child1   | 456      |
+----+----------+----------+

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父无遂子意

public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
}

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@Transactional(propagation = Propagation.NOT_SUPPORTED)
public void saveChildren() {
    saveChild1();
    int i = 1 / 0;
    saveChild2();
}

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本来就不想以事务的方式执行此方法，如果当前没有事务，岂不正合我意，于是parent、child1都没哟回滚

+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 44 | parent   | 123      |
| 45 | child1   | 456      |
+----+----------+----------+

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俚语

NOT_SUPPORT：不管老板有没有饭，我都不吃，我是个只爱干活的机器

NEVER——一说吃饭就罢工

/**
	 * Execute non-transactionally, throw an exception if a transaction exists.
	 * Analogous to EJB transaction attribute of the same name.
	 */
	NEVER(TransactionDefinition.PROPAGATION_NEVER),

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以非事务方式执行此方法，如果当前有事务直接异常

父无子无事

public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
}

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@Transactional(propagation = Propagation.NEVER)
public void saveChildren() {
    saveChild1();
    int i = 1 / 0;
    saveChild2();
}

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此情景和不用事务效果一样

+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 46 | parent   | 123      |
| 47 | child1   | 456      |
+----+----------+----------+

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父有子罢工

@Transactional(propagation = Propagation.REQUIRED)
public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();
}

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@Transactional(propagation = Propagation.NEVER)
public void saveChildren() {
    saveChild1();
    int i = 1 / 0;
    saveChild2();
}

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调用子方法时因为当前有事务，因此子方法直接抛出异常，parent的插入回滚，子方法没有执行自然没有插入数据

mysql> select * from person;
Empty set (0.00 sec)

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俚语

NEVER：老板一提吃饭，我就不干了。

NESTED

/**
	 * Execute within a nested transaction if a current transaction exists,
	 * behave like {@code REQUIRED} otherwise. There is no analogous feature in EJB.
	 * <p>Note: Actual creation of a nested transaction will only work on specific
	 * transaction managers. Out of the box, this only applies to the JDBC
	 * DataSourceTransactionManager. Some JTA providers might support nested
	 * transactions as well.
	 * @see org.springframework.jdbc.datasource.DataSourceTransactionManager
	 */
	NESTED(TransactionDefinition.PROPAGATION_NESTED);

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如果当前没有事务，那么效果同REQUIRED；否则以当前事务嵌套事务的方式执行此方法。外层事务的回滚会导致内层事务回滚（即使内层事务正常执行）。

@Transactional(propagation = Propagation.REQUIRED)
public void savePersons() {
    Person person = new Person();
    person.setUsername("parent");
    person.setPassword("123");
    personMapper.insertSelective(person);

    personService2.saveChildren();

    int i = 1 / 0;
}

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@Transactional(propagation = Propagation.NESTED)
public void saveChildren() {
    saveChild1();
    saveChild2();
}

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虽然子方法以嵌套事务的方式正常执行，但嵌套事务的操作要等当前事务模型中最外层事务的提交一并写库，否则会跟随外层事务一同回滚。这点要和REQUIRED_NEW区分开，REQUIRED_NEW是挂起当前事务另建新事务（两事务互不影响），而非在当前事务下建嵌套事务（嵌套事务受当前事务的牵制）。

因此，内层事务的提交会和外层事务一同回滚：

mysql> select * from person;
Empty set (0.00 sec)

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俚语

NESTED：老板没饭吃，自己买饭吃，想吃啥吃啥；老板有饭吃，跟着老板吃，吃啥得看老板心情