catalogue
1, Annotation (also annotated)
■ comments and meta comments acting on the code:
□ comments applied to the code:
□ meta annotation (annotation applied to other annotations):
□ highlight @ Retention and @ Target:
❀ important application of Annotation: using Annotation in reflection
(2) Annotation on method name:
Summary: use of custom annotations
1. Define an annotation class:
1, Annotation (also annotated)
Classes, methods, variables, parameters and packages in Java can be labeled. Unlike Javadoc, Java annotations can obtain annotation content through reflection. (get annotation content by reflection: when the compiler generates class files, annotations can be embedded into bytecode. Java virtual machine can keep annotation content and get annotation content at run time.)
■ annotations include: built-in annotations and custom annotations
Built in annotations: Java defines a set of seven annotations, three of which are in Java Lang, the remaining four are in Java Lang.annotation.
■ comments and meta comments acting on the code:
□ comments applied to the code:
- @Override - checks whether the method is an overridden method. If it is found that its parent class or the referenced interface does not have this method, a compilation error will be reported.
- @Deprecated - marks obsolete methods. If this method is used, a compilation warning will be reported.
- @SuppressWarnings - instructs the compiler to ignore warnings declared in annotations.
□ meta annotation (annotation applied to other annotations):
- @Retention - identifies how the annotation is saved, whether it is only in the code, incorporated into the class file, or can be accessed through reflection at run time.
- @Documented - marks whether these annotations are included in the user document.
- @Target - marks which Java member this annotation should be.
- @Inherited - marks the annotation class to which the annotation is inherited (the default annotation does not inherit from any subclasses)
□ highlight @ Retention and @ Target:
@Retention: The reserved range of annotation is an enumeration with the following optional values
RetentionPolicy.SOURCE:Annotations exist in the source file
RetentionPolicy.CLASS:Annotations exist in the source bytecode file
RetentionPolicy.RUNTIME:Annotations exist at run time
@Target: Where the annotation appears(For example, on classes, fields, methods, etc)，It is also an enumeration with the following optional values
ElementType.ANNOTATION _ type Applies to annotation types.
ElementType.CONSTRUCTOR Apply to constructor.
ElementType.FIELD Apply to a field or property.
ElementType.LOCAL_VARIABLE Apply to local variables.
ElementType.METHOD Applies to method level annotations.
ElementType.PACKAGE Apply to package declaration.
ElementType.PARAMETER Parameters applied to the method.
ElementType.TYPE Any element applied to the class.❀ important application of Annotation: using Annotation in reflection
(1) Annotation on class name:
1):@Annotation name("Annotation value) : If the annotation is on the class name, the class has the information in the annotation
@Table("t_student")
public class Student{
......
}
2): Then create an annotation:
●Where to post: @Target(ElementType.Type)
●Save time: @Retention(RetentionPolicy.RUNTIME)
public @interface Table {
String value(); //When @ Table annotation is used: @ Table(value = "t_student");
}
3): (This class has the information in the annotation) use reflection to obtain annotation information:
//Create annotation object: get the annotation object on the Student class
① Create annotation object Table table = Student class.getAnnotation(Table.class);
② Call the method of the annotation object String tableName =table.value();(2) Annotation on method name:
1)student Methods in classes test Note on:
@MyAnnotation(value = "123",name = "lisi")
public static void test(){
......
}
2)Then create an annotation:
●Where to post: @Target(ElementType.Type)
●Save time:@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
String value() default "abc";
String name() default "zhangsan";
}
3)(The method of this class has the information in the annotation) use reflection to obtain annotation information:
Class clazz = Class.forName("Student Path of");
Method[] ms = clazz.getMethods();
for (Method m : ms) {
if(m.isAnnotationPresent(MyAnnotation.class)){
//Create annotation object: get the annotation object on the test method; And call the method of the annotation object
String value = m.getAnnotation(MyAnnotation.class).value();
String name = m.getAnnotation(MyAnnotation.class).name();
}
}(3) Annotation on Attribute:
1)student Properties in class id Note on:
@Sid(min = 6,max = 10)
private int id；
2)Then create an annotation:
●Where to post: @Target(ElementType.Type)
●Save time:@Retention(RetentionPolicy.RUNTIME)
public @interface Sid{
int min() default 1;
int max() default 10;
}
3)(The attribute of this class has the information in the annotation) use reflection to obtain annotation information:
Class clazz = Class.forName("Student Path of");
Field[] fs = clazz.getDeclaredFields();
for (Field f: fs){
//Create annotation object: get @ Sid annotation on attribute
Sid id = f.getAnnotation(Test.class);
if(id != null){
f.setAccessible(true);//Set properties accessible
//Call the method of the annotation object
int min = id.min();
int max = id.max();
}
}Summary: use of custom annotations
❀ annotation is essentially an abstract class [understand it as so easy for class customization and use]
1. Define an annotation class:
@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME)
public @interface Table{
String value();
} ● @ Target(ElementType.TYPE) is a meta annotation, indicating a custom annotation class. The scope of Table annotation is TYPE class.
● @ Retention(RetentionPolicy.RUNTIME) is also a meta annotation, indicating that the custom annotation class Table annotation takes effect when RUNTIME is running.
2. Use the annotation ('paste it ') [because the annotation scope of the defined annotation is class, it can only be pasted on class]
@Table("t_stu") //It should have been written as @ Table(value="t_stu"), because the custom annotation class has only one method, abbreviated as
public class Student{
}
//===============================================================================
//There are several methods in the custom annotation, such as:
@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME)
public @interface Table{
String value();
String name();
}
//Label
@Table(value="fupo" name="xiaoming")
public class Student{
}3. Get the value of the custom annotation label, that is, call the method in the custom annotation class
● create a user-defined annotation object and call the method with the user-defined annotation object
//Get the value in the annotation label: //(1) Create the annotation object first Table table = classType.getAnnotation(Table.class); //Because the annotation is on the class Getannotation (annotation label class)) //If annotation is in attribute, use attribute Getannotation (annotation class) //(2) Call the method defined in the annotation label class to obtain the value passed in the annotation label String tableName = table.value();