Java interview questions summarized from many years of development experience - (7)

1. Hibernate pessimistic locking and optimistic locking

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Hibernate pessimistic lock: Lock the data when it is loaded. Until the lock is released, other users can make modifications. Advantages: The consistency of the data is maintained very well. Disadvantages : Not suitable for concurrent access by multiple users. When a locked resource is not released, the resource will never be modified by other users, easily causing indefinite waiting.

Hibernate optimistic locking: When the data is modified, the data is compared with the version or timestamp to see whether the data is consistent to achieve locking. The advantages are relatively good.

2. Three states of Hibernate

Temporary state: the new object has not yet been persisted and is not in Session yet

Persistent state: Already Persistence, added to the session cache, the object in this state is called a persistent object;

Free state: the persistent object is separated from the Session object. Such as objects whose Session cache has been cleared. Features: Already persisted, but not in the Session cache. Objects in this state are called free objects;

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3. The difference between hibernate and ibatis

ibatis: Open source projects are easy to get started, flexible in development, and have a heavy development workload. Most of them write their own SQL and many configuration files.

Hibernate: Open source object-relational mapping framework, high development efficiency, but cannot interfere with sql, the degree of optimization is low

4. Talk about mybatis connection pool

The common mybatis connection pools include native, c3p0, and dbcp, which are created through factory mode DataSource interface, its implementation includes unpooledDataSource (data source without connection pool), PooledDataSource (data source with connection pool), they can be obtained through the corresponding factory class object;

Take PooledDataSource as an example First: you need an object to connect to the database, and get the java.sql.Connection connection object when executing the SQL statement

Second: the PooledDataSource data source wraps the Connection pool object into a PooledConnection object and puts it in a PoolState type container maintain. MyBatis divides the connection pool in the connection pool into two states: idle state (idle) and active state (active). PooledConnection objects are stored in two List collections, idleConnections and activeConnections, in the PoolState container:

In the idle state, the PooledConnection object is placed in the idleConnections collection, indicating the currently idle PooledConnection collection that is not used. When the getConnection() method of PooledDataSource is called, the PooledConnection object will be taken first from this collection. When a java.sql.Connection object is used up, MyBatis will wrap it into a PooledConnection object and put it in this collection.

In the active state, the PooledConnection object is placed in the ArrayList named activeConnections, indicating the PooledConnection collection currently being used. When the getConnection() method of PooledDataSource is called, it will be taken from the idleConnections collection first. PooledConnection object, if not, check whether the collection is full. If not, PooledDataSource will create a PooledConnection, add it to this collection, and return;

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5. The working principle of SpringMVC

The user sends a request, which is captured and intercepted by the front-end controller DispatcherServlet;

DispatcherServlet calls HandlerMapping for processing The handler mapping management object obtains the Handler processor;

DispatcherServlet obtains the appropriate adapter HandlerAdpter according to the Handler, and the HttpMessageConveter converts the request information into the specified response object;

With the adapter, fill in the request parameters After reaching the Handler, spring begins to execute the Handler (Controller) for data conversion, data verification, and data formatting operations

After the Handler is executed, a ModelAndView object is returned to the DispatcherServlet;

According to the returned ModelAndView Choose a suitable ViewResolver view resolver and find the view specified by ModelAndView;

ViewResolver combines Model and View to render the display page;

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