annotation(注解) 解析与实例一

What is annotation?

Annotations, a form of metadata, provide data about a program that is not part of the program itself. Annotations have no direct effect on the operation of the code they annotate.

常见用途

Information for the compiler — Annotations can be used by the compiler to detect errors or suppress warnings.

比如我们常见的

@Override

,

@SuppressWarnings

编译器通过注解，去检测错误或者抑制warning.

Compile-time and deployment-time processing — Software tools can process annotation information to generate code, XML files, and so forth.

编译时部署时通过工具处理注解，以方便生产代码。

Runtime processing — Some annotations are available to be examined at runtime.

运行时检测注解

具体使用

Compiler 检测@得知它后边跟了注解。

注解可以跟元素elements,元素(name-value形式)。

如果注解中仅有一个元素，则可以直接写value。

@SuppressWarnings("unchecked")

如果注解中没有元素，则可以直接省略。

@EBook

有重复类型的注解，被称为重复注解(Repeating annotations) ，jdk1.8引入。

@Author(name = "Jane Doe")
@Author(name = "John Smith")

示例 ：

@Author(
   name = "Benjamin Franklin",
   date = "3/27/2003",
)
class MyClass() { ... }

@SuppressWarnings("unchecked")
void myMethod() { ... }

@Author(name = "Jane Doe")
@EBook
class MyClass { ... }

@Author(name = "Jane Doe")
@Author(name = "John Smith")
class MyClass { ... }

对于注解的类型，都是在java.lang.annotation中定义，预定义的注解类型有：

java.lang.Deprecated

(implements java.lang.annotation.Annotation)： 该方法（或类、接口等等）弃用。(使用时，注意javadoc中用 @deprecated)

public class Snippet { 
    // Javadoc comment follows
    /**
     * @deprecated explanation of why it was deprecated
     */
    @Deprecated
    static void deprecatedMethod() {
    }
}

java.lang.@Override (implements java.lang.annotation.Annotation)

编译器通过这个注释去检测重写的正确性。

java.lang.@SuppressWarnings (implements java.lang.annotation.Annotation)用于抑制代码中提示的warning信息。

// use a deprecated method and tell 
   // compiler not to generate a warning
   @SuppressWarnings("deprecation")
    void useDeprecatedMethod() {
        // deprecation warning
        // - suppressed
        objectOne.deprecatedMethod();
    }

java.lang.@SafeVarargs (implements java.lang.annotation.Annotation)

用于抑制可变参数使用报出的warning信息.

java.lang.FunctionalInterface (implements java.lang.annotation.Annotation)

jdk1.8引入，声明功能性接口。

元注解:用于声明其他注解的注解

java.lang.annotation.@Retention:表示如何保留被标识的注解：

RetentionPolicy.SOURCE – The marked annotation is retained only in the source level and is ignored by the compiler.

源码级别注释保留，被 java compiler 忽略编译。

RetentionPolicy.CLASS – The marked annotation is retained by the compiler at compile time, but is ignored by the Java Virtual Machine (JVM).

java compiler 编译时保留注解。被JVM忽略。

RetentionPolicy.RUNTIME – The marked annotation is retained by the JVM so it can be used by the runtime environment.

运行时保留该注解，被JVM保留。

java.lang.annotation.@Documented：

通过javadoc tool 处理该注释，仅作为文档。

java.lang.annotation.@Target ：

用来表示被标识的注解用于指定的类型。

ElementType.ANNOTATION_TYPE can be applied to an annotation type.

ElementType.CONSTRUCTOR can be applied to a constructor.

ElementType.FIELD can be applied to a field or property.

ElementType.LOCAL_VARIABLE can be applied to a local variable.

ElementType.METHOD can be applied to a method-level annotation.

ElementType.PACKAGE can be applied to a package declaration.

ElementType.PARAMETER can be applied to the parameters of a method.

ElementType.TYPE can be applied to any element of a class.

java.lang.annotation@Inherited

表示这个注解可以继承超类注解。注意：这个注解只能用来声明类

示例：

package inherited.test;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Inherited
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface InheritedAnnotationType {

}

package inherited.test;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface UninheritedAnnotationType {

}

package inherited.test;
@UninheritedAnnotationType
class A {

}

package inherited.test;
@InheritedAnnotationType
class B extends A {

}

package inherited.test;
class C extends B {

}

/**
 * 
 */
package inherited.test;

public class Test {
    public static void main(String[] args) {

    System.out.println(new A().getClass().getAnnotation(InheritedAnnotationType.class));
    System.out.println(new B().getClass().getAnnotation(InheritedAnnotationType.class));
    System.out.println(new C().getClass().getAnnotation(InheritedAnnotationType.class));
    System.out.println("_________________________________");
    System.out.println(new A().getClass().getAnnotation(UninheritedAnnotationType.class));
    System.out.println(new B().getClass().getAnnotation(UninheritedAnnotationType.class));
    System.out.println(new C().getClass().getAnnotation(UninheritedAnnotationType.class));
}
}

output:

null
@inherited.test.InheritedAnnotationType()
@inherited.test.InheritedAnnotationType()
_________________________________
@inherited.test.UninheritedAnnotationType()
null
null