Detailed explanation of the annotation framework commonly used in the Java class library

Detailed explanation of the annotation framework commonly used in the Java class library Overview: In the Java library, Annotion is a metadata that can be used to provide instructions, statements and marks of various elements in the program.The annotation framework is a collection of tools for defining and processing annotations. It can simplify the writing and maintenance of code and improve the readability and maintenance of the code.In this article, we will introduce the annotation framework and its usage methods in some commonly used Java libraries in detail. 1. Comment in the Junit4 framework Junit4 is one of the most commonly used unit test frameworks in Java, which uses many annotations to mark the relevant information of test cases.The following are the most commonly used annotations and usage in Junit4: 1. @Test: Used to mark a method as a test method.Each test method should be marked as @test so that it can be automatically identified and executed by the Junit4 framework. @Test public void testAdd() { // Test code // Ecclail results } 2. @Before: Used to mark a method, automatically execute before each test method, and use it to do some initialization operations. @Before public void setUp() { // Initialize operation } 3. @AFTER: Used to mark a method, automatically execute after each test method, and use it to do some cleaning operations. @After public void tearDown() { // Cleany operation } 4. @BefaceClass: Used to mark a static method, automatically execute before the entire test class is performed to do some global initialization operations. @BeforeClass public static void setUpClass() { // Global initialization operation } 5. @AterClass: Used to mark a static method, automatically execute after the entire test class is performed to do some global cleaning operations. @AfterClass public static void tearDownClass() { // Global cleanup operation } 2. Comment in the Spring framework The Spring framework provides many annotations for simplifying configuration and managing JavaBean.The following are the most commonly used annotations and usage in the Spring framework: 1. @component: Used to mark a class as a component (that is, JavaBean), the Spring framework will automatically scan and register it into the container for other classes. @Component public class UserService { // Class content } 2. @Autowired: Tell the container to automatically inject dependent objects in the member variables, constructive methods, or methods for labeling. @Component public class UserController { @Autowired private UserService userService; // Class content } 3. @qualifier: Used with @Autowired annotation. When there are multiple objects of the same type, specify which object is specified. @Component @Qualifier("UserServiceImpl") public class UserServiceImpl implements UserService { // Class content } 4. @ReastController: Used to mark a class as a controller for handling HTTP requests and responses. @RestController public class UserController { // Class content } 5. @RequestMapping: URL paths and HTTP request methods used to mark the controller method. @RestController @RequestMapping("/users") public class UserController { @GetMapping("/{id}") public User getUser(@PathVariable int id) { // Processing the logic of obtaining users } } Third, annotations in the Hibernate framework Hibernate is an open source object relationship mapping (ORM) framework to simplify the mapping between the Java object and the relationship database.The following are the most commonly used annotations and usage in the Hibernate framework: 1. @entity: Used to mark a class as a physical class and map it to the database. @Entity @Table(name = "users") public class User { // Class content } 2. @table: For the relevant information of the database table mapped by the physical class, such as table names, indexes, etc. @Entity @Table(name = "users") public class User { // Class content } 3. @ID: Used to mark a attribute as the primary key. @Entity @Table(name = "users") public class User { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private int id; // Class content } 4. @Column: The relevant information used to mark a field that attributes are mapping to the database table, such as field name, type, length, etc. @Entity @Table(name = "users") public class User { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private int id; @Column(name = "username") private String username; // Class content } 5. @onetomany: Used to mark a pair of multi -relationship between one attribute and another physical class. @Entity @Table(name = "users") public class User { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private int id; @OneToMany(mappedBy = "user") private List<Order> orders; // Class content } Summarize: This article introduces the commonly used annotation framework and its usage methods in the Java library, including common annotations in the Junit4 framework, common annotations in the Spring framework, and commonly used annotations in the Hibernate framework.These frameworks provide rich annotations to help us simplify the writing and maintenance of code, and improve the readability and maintenance of code.I hope this article will be helpful to the annotation framework in the Java class library.