Interview Question: Affairs

Second, sql optimization: (index, paradigm)

Paradigm:

First Normal Form (ensuring atomicity of each column) is the most basic normal form. All field values in the database table are indecomposable atomic values, which satisfy the first normal form.

Second Normal Form (ensuring that every column in the table is associated with the primary key) is one level closer to First Normal Form. Make sure that each column in the database table is related to the primary key, not only a certain part of the primary key. That is to say, only one type of data can be stored in one table, and multiple types of data cannot be stored in one table.

Third Normal Form: Make sure that each column is directly related to the primary key column, not indirectly related.

index:

Avoid doing calculations on indexed fields, and avoid using not, <>, and ! =, Avoid using IS NULL and NOT NULL on indexes, Avoid data type conversion on indexed columns, Avoid using functions on indexed fields, Avoid NULL values in indexed columns

Steps for usage:

Step 1. Introduce the <tx:> namespace
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001 in the spring configuration file /XMLSchema-instance"
xmlns:tx="http://www.springframework.org/schema/tx"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework. org/schema/beans/spring- beans -2.0.xsd
http://www.springframework.org/schema/tx
http://www.springframework.org/schema/tx/spring-tx-2.0.xsd">step

2. Beans annotated with @Transactional are automatically configured for declarative transaction support

<!-- Transaction manager configuration, Hibernate single data source transaction -->
    <bean id="defaultTransactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager">
        <property name="sessionFactory" ref="sessionFactory" />
    </bean>
    
    <!-- Use annotation to define transactions-->
    <tx:annotation-driven transaction-manager="defaultTransactionManager" proxy-target-class="true" />

Step 3. Write a @Transactional at the declaration of the interface or class.
If you only write it on the interface, the implementation class of the interface will be inherited. The specific method of the implementation class of the interface can override the setting
@Transactional / at the declaration of the class. /Class-level annotations, applicable to all public methods in the class

Transaction propagation behavior and isolation levels

When you use spring's annotated transaction management, you may be a little overwhelmed with the propagation behavior and isolation level of transactions. The following is a detailed introduction for easy reference.

Thing annotation method: @Transactional

When marked before a class, all methods in the marked class are processed, for example:

@Transactional
public class TestServiceBean implements TestService {}

When some methods in a class don't need things:

@Transactional
public class TestServiceBean implements TestService {   
    private TestDao dao;   
    public void setDao(TestDao dao) {
        this.dao = dao;
    }   
   @Transactional(propagation = Propagation.NOT_SUPPORTED)
    public List<Object> getAll() {
        return null;
    }   
}

Introduction of transaction propagation behavior:
@Transactional(propagation=Propagation.REQUIRED)
If there is a transaction, then join the transaction, if not, create a new one (by default)
@Transactional(propagation=Propagation.NOT_SUPPORTED)
The container does not open a transaction for this method
@Transactional( propagation=Propagation.REQUIRES_NEW)
Regardless of whether there is a transaction, a new transaction is created, the original is suspended, the new one is executed, and the old transaction is continued.
@Transactional(propagation=Propagation.MANDATORY)
must be in an existing transaction Execute, otherwise throw exception
@Transactional(propagation=Propagation.NEVER)
must be executed in a transaction that does not have, otherwise throw exception (as opposed to Propagation.MANDATORY)
@Transactional(propagation=Propagation.SUPPORTS)
if other beans call this method , declare transactions in other beans, then use transactions. If other beans do not declare transactions, then do not use transactions.

Transaction timeout setting:
@Transactional(timeout=30) //The default is 30 seconds

Transaction isolation level:
@Transactional(isolation = Isolation.READ_UNCOMMITTED)
reads uncommitted data (dirty read, non-repeatable read) basically does not use
@Transactional(isolation = Isolation.READ_COMMITTED)
to read committed data (non-repeatable will appear ) Read and phantom read)
@Transactional(isolation = Isolation.REPEATABLE_READ)
Repeatable read (phantom read)
@Transactional(isolation = Isolation.SERIALIZABLE)
Serialization

MYSQL: default to REPEATABLE_READ level
SQLSERVER: default to READ_COMMITTED

Dirty read : One transaction reads the uncommitted update data of another transaction.
Non-repeatable read : In the same transaction, reading the same data multiple times returns different results, in other words,
subsequent reads can read another The updated data submitted by the transaction. On the contrary, when "repeatable read" reads data multiple times in the same transaction , it can ensure that the data read is the same, that is, subsequent reads cannot read
the updated data submitted by another transaction. : A transaction reads the committed insert data of another transaction

Description of common parameters in @Transactional annotation

parameter name

Function description

readOnly

This property is used to set whether the current transaction is a read-only transaction. When set to true, it means read-only, and false means that it can be read and written. The default value is false. For example: @Transactional(readOnly=true)

rollbackFor

This property is used to set the exception class array that needs to be rolled back. When the exception in the specified exception array is thrown in the method, the transaction will be rolled back. E.g:

Specify a single exception class: @Transactional(rollbackFor=RuntimeException.class)

Specify multiple exception classes: @Transactional(rollbackFor={RuntimeException.class, Exception.class})

continued)

parameter name	Function description
rollbackForClassName	This property is used to set an array of exception class names that need to be rolled back. When an exception in the specified exception name array is thrown in the method, the transaction will be rolled back. E.g: Specify a single exception class name: @Transactional(rollbackForClassName="RuntimeException") Specify multiple exception class names: @Transactional(rollbackForClassName={"RuntimeException","Exception"})
noRollbackFor	This property is used to set the exception class array that does not need to be rolled back. When the exception in the specified exception array is thrown in the method, the transaction will not be rolled back. E.g: Specify a single exception class: @Transactional(noRollbackFor=RuntimeException.class) Specify multiple exception classes: @Transactional(noRollbackFor={RuntimeException.class, Exception.class})
noRollbackForClassName	This property is used to set an array of exception class names that do not need to be rolled back. When an exception in the specified exception name array is thrown in the method, the transaction will not be rolled back. E.g: Specify a single exception class name: @Transactional(noRollbackForClassName="RuntimeException") Specify multiple exception class names: @Transactional(noRollbackForClassName={"RuntimeException","Exception"})
propagation	This property is used to set the propagation behavior of the transaction. For specific values, refer to Table 6-7. 例如：@Transactional(propagation=Propagation.NOT_SUPPORTED,readOnly=true)
isolation	This property is used to set the transaction isolation level of the underlying database. The transaction isolation level is used to handle multi-transaction concurrency. Usually, the default isolation level of the database can be used, and there is basically no need to set it.
timeout	This property is used to set the timeout seconds of the transaction. The default value is -1, which means never timeout.

注意的几点:
1 @Transactional 只能被应用到public方法上, 对于其它非public的方法,如果标记了@Transactional也不会报错,但方法没有事务功能.

2用 spring 事务管理器,由spring来负责数据库的打开,提交,回滚.默认遇到运行期例外(throw new RuntimeException("注释");)会回滚，即遇到不受检查（unchecked）的例外时回滚；而遇到需要捕获的例外(throw new Exception("注释");)不会回滚,即遇到受检查的例外（就是非运行时抛出的异常，编译器会检查到的异常叫受检查例外或说受检查异常）时，需我们指定方式来让事务回滚要想所有异常都回滚,要加上 @Transactional( rollbackFor={Exception.class,其它异常}) .如果让unchecked例外不回滚： @Transactional(notRollbackFor=RunTimeException.class)
如下:
@Transactional(rollbackFor=Exception.class) //指定回滚,遇到异常Exception时回滚
public void methodName() {
throw new Exception("注释");

}
@Transactional(noRollbackFor=Exception.class)//指定不回滚,遇到运行期例外(throw new RuntimeException("注释");)会回滚
public ItimDaoImpl getItemDaoImpl() {
throw new RuntimeException("注释");
}

3、@Transactional 注解应该只被应用到 public 可见度的方法上。如果你在 protected、private 或者 package-visible 的方法上使用 @Transactional 注解，它也不会报错，但是这个被注解的方法将不会展示已配置的事务设置。

4、@Transactional 注解可以被应用于接口定义和接口方法、类定义和类的 public 方法上。然而，请注意仅仅 @Transactional 注解的出现不足于开启事务行为，它仅仅是一种元数据，能够被可以识别 @Transactional 注解和上述的配置适当的具有事务行为的beans所使用。上面的例子中，其实正是 <tx:annotation-driven/>元素的出现开启了事务行为。

5、Spring团队的建议是你在具体的类（或类的方法）上使用 @Transactional 注解，而不要使用在类所要实现的任何接口上。你当然可以在接口上使用 @Transactional 注解，但是这将只能当你设置了基于接口的代理时它才生效。因为注解是不能继承的，这就意味着如果你正在使用基于类的代理时，那么事务的设置将不能被基于类的代理所识别，而且对象也将不会被事务代理所包装（将被确认为严重的）。因此，请接受Spring团队的建议并且在具体的类上使用 @Transactional 注解。

使用步骤：

步骤一、在spring配置文件中引入<tx:>命名空间
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:tx="http://www.springframework.org/schema/tx"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/tx
http://www.springframework.org/schema/tx/spring-tx-2.0.xsd">

步骤二、具有@Transactional 注解的bean自动配置为声明式事务支持

<!-- 事务管理器配置, Hibernate单数据源事务 -->
    <bean id="defaultTransactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager">
        <property name="sessionFactory" ref="sessionFactory" />
    </bean>
    
    <!-- 使用annotation定义事务 -->
    <tx:annotation-driven transaction-manager="defaultTransactionManager" proxy-target-class="true" />

步骤三、在接口或类的声明处 ,写一个@Transactional.
要是只在接口上写, 接口的实现类就会继承下来、接口的实现类的具体方法,可以覆盖类声明处的设置
@Transactional //类级的注解、适用于类中所有的public的方法

事务的传播行为和隔离级别

大家在使用spring的注解式事务管理时，对事务的传播行为和隔离级别可能有点不知所措，下边就详细的介绍下以备方便查阅。

事物注解方式: @Transactional

当标于类前时, 标示类中所有方法都进行事物处理 , 例子:

@Transactional
public class TestServiceBean implements TestService {}

当类中某些方法不需要事物时:

@Transactional
public class TestServiceBean implements TestService {   
    private TestDao dao;   
    public void setDao(TestDao dao) {
        this.dao = dao;
    }   
    @Transactional(propagation = Propagation.NOT_SUPPORTED)
    public List<Object> getAll() {
        return null;
    }   
}

事物传播行为介绍:
@Transactional(propagation=Propagation.REQUIRED)
如果有事务, 那么加入事务, 没有的话新建一个(默认情况下)
@Transactional(propagation=Propagation.NOT_SUPPORTED)
容器不为这个方法开启事务
@Transactional(propagation=Propagation.REQUIRES_NEW)
不管是否存在事务,都创建一个新的事务,原来的挂起,新的执行完毕,继续执行老的事务
@Transactional(propagation=Propagation.MANDATORY)
必须在一个已有的事务中执行,否则抛出异常
@Transactional(propagation=Propagation.NEVER)
必须在一个没有的事务中执行,否则抛出异常(与Propagation.MANDATORY相反)
@Transactional(propagation=Propagation.SUPPORTS)
如果其他bean调用这个方法,在其他bean中声明事务,那就用事务.如果其他bean没有声明事务,那就不用事务.

事物超时设置:
@Transactional(timeout=30) //默认是30秒

事务隔离级别:
@Transactional(isolation = Isolation.READ_UNCOMMITTED)
读取未提交数据(会出现脏读, 不可重复读) 基本不使用
@Transactional(isolation = Isolation.READ_COMMITTED)
读取已提交数据(会出现不可重复读和幻读)
@Transactional(isolation = Isolation.REPEATABLE_READ)
可重复读(会出现幻读)
@Transactional(isolation = Isolation.SERIALIZABLE)
串行化

MYSQL: 默认为REPEATABLE_READ级别
SQLSERVER: 默认为READ_COMMITTED

脏读 : 一个事务读取到另一事务未提交的更新数据
不可重复读 : 在同一事务中, 多次读取同一数据返回的结果有所不同, 换句话说,
后续读取可以读到另一事务已提交的更新数据. 相反, "可重复读"在同一事务中多次
读取数据时, 能够保证所读数据一样, 也就是后续读取不能读到另一事务已提交的更新数据
幻读 : 一个事务读到另一个事务已提交的insert数据

@Transactional注解中常用参数说明

参数名称

功能描述

readOnly

该属性用于设置当前事务是否为只读事务，设置为true表示只读，false则表示可读写，默认值为false。例如：@Transactional(readOnly=true)

rollbackFor

该属性用于设置需要进行回滚的异常类数组，当方法中抛出指定异常数组中的异常时，则进行事务回滚。例如：

指定单一异常类：@Transactional(rollbackFor=RuntimeException.class)

指定多个异常类：@Transactional(rollbackFor={RuntimeException.class, Exception.class})

续表）

参数名称	功能描述
rollbackForClassName	该属性用于设置需要进行回滚的异常类名称数组，当方法中抛出指定异常名称数组中的异常时，则进行事务回滚。例如：指定单一异常类名称：@Transactional(rollbackForClassName="RuntimeException") 指定多个异常类名称：@Transactional(rollbackForClassName={"RuntimeException","Exception"})
noRollbackFor	该属性用于设置不需要进行回滚的异常类数组，当方法中抛出指定异常数组中的异常时，不进行事务回滚。例如：指定单一异常类：@Transactional(noRollbackFor=RuntimeException.class) 指定多个异常类：@Transactional(noRollbackFor={RuntimeException.class, Exception.class})
noRollbackForClassName	该属性用于设置不需要进行回滚的异常类名称数组，当方法中抛出指定异常名称数组中的异常时，不进行事务回滚。例如：指定单一异常类名称：@Transactional(noRollbackForClassName="RuntimeException") 指定多个异常类名称： @Transactional(noRollbackForClassName={"RuntimeException","Exception"})
propagation	该属性用于设置事务的传播行为，具体取值可参考表6-7。例如：@Transactional(propagation=Propagation.NOT_SUPPORTED,readOnly=true)
isolation	该属性用于设置底层数据库的事务隔离级别，事务隔离级别用于处理多事务并发的情况，通常使用数据库的默认隔离级别即可，基本不需要进行设置
timeout	该属性用于设置事务的超时秒数，默认值为-1表示永不超时

3. The @Transactional annotation should only be applied to methods with public visibility. If you use the @Transactional annotation on a protected, private or package-visible method, it will also not throw an error, but the annotated method will not display the configured transaction settings.

4. The @Transactional annotation can be applied to interface definitions and interface methods, class definitions and public methods of classes. However, please note that the mere presence of the @Transactional annotation is not sufficient to enable transactional behavior, it is merely metadata that can be used by beans that recognize the @Transactional annotation and configure appropriately transactional behavior as described above. In the above example, it is the appearance of the <tx:annotation-driven/> element that enables the transactional behavior.

5. The recommendation of the Spring team is that you use the @Transactional annotation on a specific class (or method of a class), rather than any interface that the class wants to implement. You can of course use the @Transactional annotation on an interface, but this will only work if you have an interface-based proxy set up. Because annotations are not inheritable, this means that if you are using class-based proxies, then the transaction settings will not be recognized by class-based proxies, and objects will not be wrapped by transactional proxies (will be confirmed serious). Therefore, please take the advice of the Spring team and use the @Transactional annotation on concrete classes.

Interview Question: Affairs

Guess you like