2020 New Java Interview Questions-Spring (2)

#1. Are beans in spring thread-safe?

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Whether the beans in the Spring container are thread-safe, the container itself does not provide a thread-safety policy for the beans, so It can be said that the beans in the spring container themselves do not have thread safety characteristics, but the details still need to be studied in combination with the beans in the specific scope.

2. How many bean scopes does spring support?

When you create a Bean instance through the spring container, you can not only complete the instantiation of the Bean instance, but also specify a specific scope for the Bean. Spring supports the following five scopes:

singleton: singleton mode. In the entire Spring IoC container, beans defined using singleton will have only one instance.

prototype: prototype mode, each time it is passed When the getBean method of the container obtains the Bean defined by prototype, a new Bean instance will be generated

request: For each HTTP request, the Bean defined using request will generate a new instance, that is, each HTTP request Different Bean instances will be generated. This scope is only valid when using Spring in a web application

session: For each HTTP Session, a new instance is generated using the Bean beans defined by the session. This scope is also only valid when using Spring in a web application

globalsession: For each global HTTP Session, the beans defined using the session will generate a new instance. Typically, this is only valid when using a portlet context. This scope is also only valid when using Spring in a web application

The more commonly used scopes are singleton and prototype. For singleton scoped beans, you will get the same instance every time you request the bean. The container is responsible for tracking the status of the Bean instance and maintaining the life cycle behavior of the Bean instance;

If a Bean is set to prototype scope, Spring will create a new Bean instance every time the program requests the Bean with this ID, and then Return to program. In this case, the Spring container only uses the new keyword to create the Bean instance. Once created successfully, the container no longer tracks the instance and does not maintain the state of the Bean instance.

If you do not specify the scope of the bean, Spring uses the singleton scope by default.

When Java creates a Java instance, it needs to apply for memory; when it destroys the instance, it needs to complete garbage collection. These tasks will lead to an increase in system overhead. Therefore, the creation and destruction of prototype scope beans is relatively expensive. Once the Bean instance in the singleton scope is successfully created, it can be reused.

Therefore, unless necessary, try to avoid setting the Bean to the prototype scope.

3. What are the ways for spring to automatically assemble beans?

The Spring container is responsible for creating beans in the application and coordinating the relationship between these objects through IDs. As developers, we need to tell Spring which beans to create and how to wire them together.

There are two ways to assemble beans in spring:

1. Implicit bean discovery mechanism and automatic assembly

2. Explicit configuration in java code or XML

Of course these methods can also be used in conjunction.

4. What are the implementation methods of spring transactions?

Programmatic transaction management is the only option for POJO-based applications. We need to call beginTransaction(), commit(), rollback() and other transaction management-related methods in the code. This is programmatic transaction management.

Declarative transaction management based on TransactionProxyFactoryBean

Declarative transaction management based on @Transactional

Configuring transactions based on Aspectj AOP

5. Talk What about spring's transaction isolation?

Transaction isolation level refers to the degree of isolation between the modification of data by one transaction and another parallel transaction. When multiple transactions access the same data at the same time, if the necessary isolation mechanism is not adopted, it will The following problems may occur:

Dirty read: One transaction reads uncommitted update data from another transaction.

Phantom read: For example, the first transaction modifies the data in a table. For example, this modification involves "all data rows" in the table. At the same time, the second transaction also modifies the data in this table. This modification is to insert "a row of new data" into the table.

Then, in the future, the user who operates the first transaction will find that there are still unmodified data rows in the table, as if an illusion has occurred.

Non-repeatable reading: For example, two identical select statements are executed successively in the same transaction. During this period, no DDL statements were executed in this transaction, but the results obtained successively are inconsistent. This is non-repeatable. read.

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