SpringBoot自動裝配&事務傳播策略

時間  2019-12-16
標籤 springboot 自動 裝配 事務 傳播 策略 欄目 Spring 简体版
原文   原文鏈接

SpringBoot自動裝配

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}
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爲什麼使用SpringBoot後，咱們只需上述幾行代碼便可搭建一個web服務器，比以前使用SpringMVC不要簡潔太多。java

這其中奧妙在於@SpringBootApplication註解之中：mysql

@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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其又繼承了@EnableAutoConfiguration註解：web

/** * ... * 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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查看註釋可知，自動配置類也是以常規的Spring Bean的形式存在。它們被SpringFactoriesLoader定位：spring

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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該類會從類路徑中的"META-INF/spring.factories"中讀取自動裝配的類列表，其中spring-boot-autoconfigure.jar中的就包含了內嵌Tomcat、SpringMVC、事務等功能的配置類：sql

org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration
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事務傳播策略

前言

方法爲維度

因爲Spring事務是基於AOP的，因此事務以方法爲維度存在於Java代碼中。而事務的傳播是基於多事務之間相互影響的，因此在代碼中表現爲一個事務方法調用另外一個事務方法（以下列代碼中savePersons方法中調用saveChildren）：bash

@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爲入口

可是，SpringAOP是基於bean加強的，也就是說當你調用一個bean的事務方法（被事務註解修飾的方法）時，該事務註解是能夠正常生效的。但若是你調用本類中的事務方法，那就至關於將該方法中的代碼內嵌到當前方法中，即該方法的事務註解會被忽略。服務器

例如：app

@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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上列代碼等效於下列代碼（saveChildren方法事務註解被忽略掉了）：ide

@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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所以咱們接下來討論的一個事務/非事務方法調用另外一個事務/非事務方法，這兩個方法被調用的方式都是基於bean做爲方法引用的，而非經過this調用本類中的方法。所以咱們不妨將兩個寫庫方法saveChildren和savePersons移入兩個bean中進行測試：spring-boot

@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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當前事務&父方法事務

本文中說父方法是否建立事務，不單單是指調用本方法的調用方，而是泛指方法調用鏈的上游方法，只要上游方法中的任意一個方法開啓了事務，那麼當前方法的執行就處於事務之中，也即執行當前方法時存在事務。

策略枚舉

Spring事務傳播策略相關的枚舉類以下：

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——有飯就吃，沒飯本身買

@Transactional註解默認的傳播策略就是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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被該註解表示：若是當前方法有事務，則支持當前事務；若是當前方法沒有事務，則新建事務供本身使用。

父無，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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因爲父方法沒有註解，執行到第7行時，調用了傳播策略爲REQUIRED的事務方法，其本身新建事務供本身使用，所以child1, child2由於1/0異常不會被插入，異常拋至父方法，父方法由於沒有事務因此不會回滾以前插入的parent，執行結果以下：

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

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父有，子繼承

@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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因爲test調用bean personService的事務方法savePersons且其傳播策略爲REQUIRED，因而其新建一個事務給本身用，當調用bean personService2的REQUIRED事務方法時，發現當前有事務所以支持當前事務，所以parent、child一、child2的插入因爲在同一個事務中，所以在1/0異常拋出後都被回滾：

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

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俚語

REQUIRED，老闆（父方法）有飯吃（有事務），我（子方法）跟着老闆吃（支持當前事務）；老闆沒飯吃，我本身買飯吃

SUPPORTS——有飯就吃，沒飯餓肚子

/**
	 * 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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若是當前有事務，則支持當前事務，不然以非事務的方式執行當前方法

父有子支持

當父方法會建立事務時，子方法用SUPPORTS和用REQUIRED效果是同樣的

@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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父無子無

當父方法沒有建立事務，那麼子方法也不會自做主張去新建事務：

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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俚語

SUPPORTS：老闆有飯吃，我跟着老闆吃；老闆沒飯吃，我就只能餓肚子了。

MANDATORY——必需要有飯吃

/**
	 * 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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強制以事務的方式執行當前方法：若是當前有事務，那麼支持當前事務，不然拋出異常

父有子支持

這點REQUIRED, SUPPORTS, MANDATORY是同樣的

父無子罷工

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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當執行到personService2的MANDATORY事務方法時，發現當前沒有事務，因而它直接拋出一個異常：

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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俚語

MANDATORY：老闆有飯吃，跟着老闆吃；老闆沒飯吃，老子不幹了。有飯才幹活

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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無論當前有沒有事務，本身都會新建一個事務爲本身所用，而且若是當前有事務那麼就會掛起當前事務

父無子自強

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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此次我將1/0移入了父方法中，父方法有事務所以parent的插入會回滾，可是子方法的執行會掛起當前事務另建新事務，所以子方法的插入依然有效（子方法執行結束後父方法的事務又會被自動恢復）

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

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俚語

REQUIRED_NEW：無論老闆有沒有飯吃，我都本身買飯吃，不接受他人的恩惠。

NOT_SUPPORTED——不吃飯只幹活

/** * 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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強制以非事務的方式執行當前代碼，若是當前有事務則將其掛起。

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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執行子方法時，事務被掛起，所以child1的插入未被回滾，回到父方法後事務被恢復，所以parent的插入被回滾

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

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父無遂子意

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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原本就不想以事務的方式執行此方法，若是當前沒有事務，豈不正合我意，因而parent、child1都沒喲回滾

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

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俚語

NOT_SUPPORT：無論老闆有沒有飯，我都不吃，我是個只愛幹活的機器

NEVER——一說吃飯就罷工

/** * 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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以非事務方式執行此方法，若是當前有事務直接異常

父無子無事

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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此情景和不用事務效果同樣

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

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父有子罷工

@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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調用子方法時由於當前有事務，所以子方法直接拋出異常，parent的插入回滾，子方法沒有執行天然沒有插入數據

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

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俚語

NEVER：老闆一提吃飯，我就不幹了。

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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若是當前沒有事務，那麼效果同REQUIRED；不然以當前事務嵌套事務的方式執行此方法。外層事務的回滾會致使內層事務回滾（即便內層事務正常執行）。

@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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雖然子方法以嵌套事務的方式正常執行，但嵌套事務的操做要等當前事務模型中最外層事務的提交一併寫庫，不然會跟隨外層事務一同回滾。這點要和REQUIRED_NEW區分開，REQUIRED_NEW是掛起當前事務另建新事務（兩事務互不影響），而非在當前事務下建嵌套事務（嵌套事務受當前事務的牽制）。

所以，內層事務的提交會和外層事務一同回滾：

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

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俚語

NESTED：老闆沒飯吃，本身買飯吃，想吃啥吃啥；老闆有飯吃，跟着老闆吃，吃啥得看老闆心情

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