程序元素上的注解是Java对象。允许访问其注解的程序元素实现java.lang.reflect.AnnotatedElement接口。以下类实现了AnnotatedElement接口：
AnnotatedElement接口的方法用于访问以下列出的对象类型的注解。
java.lang.Class java.lang.reflect.Executable java.lang.reflect.Constructor java.lang.reflect.Field java.lang.reflect.Method java.lang.reflect.Parameter java.lang.Package java.lang.reflect.AccessibleObject
注解类型必须使用运行时的保留策略来保留元注解，以便在运行时访问它。
示例 - 1
假设有一个Test类，并且想打印它的所有注解。参考以下代码片段，它将打印Test类的类声明上的所有注解：
import java.lang.annotation.Annotation
@SuppressWarnings("unchecked")
@Deprecated
public class Main {
public static void main(String[] argv) {
// Get the class object reference
Class<Main> c = Main.class
// Get all annotations on the class declaration
Annotation[] allAnns = c.getAnnotations()
System.out.println("Annotation count: " + allAnns.length)
// Print all annotations
for (Annotation ann : allAnns) {
System.out.println(ann)
}
}
}
执行上面的代码得到如下结果 -
Annotation count: 1 @java.lang.Deprecated()
如上面的结果所示，Annotation接口的toString()方法返回注解的字符串表示形式。
实例-2
以下代码显示了如何获取指定的注解，请参考以下代码 -
import java.lang.annotation.Documented
import java.lang.annotation.Retention
import java.lang.annotation.RetentionPolicy
@Retention(RetentionPolicy.RUNTIME)
@Documented
@interface Version {
int major()
int minor()
}
@Version(major=1,minor=2)
public class Main {
public static void main(String[] argv) {
Class<Main> c = Main.class
Version v = c.getAnnotation(Version.class)
if (v == null) {
System.out.println("Version annotation is not present.")
} else {
int major = v.major()
int minor = v.minor()
System.out.println("Version: major=" + major + ", minor=" + minor)
}
}
}
上面的代码生成以下结果。
Version: major=1, minor=2
实例-3
以下代码显示了如何访问方法的注解，请参考以下代码 -
import java.lang.annotation.Annotation
import java.lang.annotation.Retention
import java.lang.annotation.RetentionPolicy
import java.lang.reflect.AnnotatedElement
import java.lang.reflect.Method
@Retention(RetentionPolicy.RUNTIME)
@interface Version {
int major()
int minor()
}
@Version(major = 1, minor = 0)
class AccessAnnotation {
@Version(major = 1, minor = 1)
public void testMethod1() {
}
@Version(major = 1, minor = 2)
@Deprecated
public void testMethod2() {
}
}
public class Main {
public static void main(String[] args) {
Class<AccessAnnotation> c = AccessAnnotation.class
System.out.println("Annotations for class:" + c.getName())
printAnnotations(c)
System.out.println("Method annotations:")
Method[] m = c.getDeclaredMethods()
for (int i = 0 i < m.length i++) {
System.out.println("Annotations for method:" + m[i].getName())
printAnnotations(m[i])
}
}
public static void printAnnotations(AnnotatedElement programElement) {
Annotation[] annList = programElement.getAnnotations()
for (int i = 0 i < annList.length i++) {
System.out.println(annList[i])
if (annList[i] instanceof Version) {
Version v = (Version) annList[i]
int major = v.major()
int minor = v.minor()
System.out.println("Found Version annotation: " + "major =" + major
+ ", minor=" + minor)
}
}
}
}
上面的代码生成以下结果。
Annotations for class:AccessAnnotation @Version(major=1, minor=0) Found Version annotation: major =1, minor=0 Method annotations: Annotations for method:testMethod1 @Version(major=1, minor=1) Found Version annotation: major =1, minor=1 Annotations for method:testMethod2 @Version(major=1, minor=2) Found Version annotation: major =1, minor=2 @java.lang.Deprecated()
实例-4
以下代码显示了如何在运行时访问可重复注解的实例，请参考以下代码 -
import java.lang.annotation.Repeatable
import java.lang.annotation.Retention
import java.lang.annotation.RetentionPolicy
@Retention(RetentionPolicy.RUNTIME)
@interface LogHistory {
Log[] value()
}
@Repeatable(LogHistory.class)
@interface Log {
String date()
String comments()
}
@Log(date = "02/01/2018", comments = "A")
@Log(date = "01/22/2018", comments = "B")
public class Main {
public static void main(String[] args) {
Class<Main> mainClass = Main.class
Log[] annList = mainClass.getAnnotationsByType(Log.class)
for (Log log : annList) {
System.out.println("Date=" + log.date() + ", Comments=" + log.comments())
}
Class<LogHistory> containingAnnClass = LogHistory.class
LogHistory logs = mainClass.getAnnotation(containingAnnClass)
for (Log log : logs.value()) {
System.out.println("Date=" + log.date() + ", Comments=" + log.comments())
}
}
}
上面的代码生成以下结果。
Date=02/01/2018, Comments=A Date=01/22/2018, Comments=B Date=02/01/2018, Comments=A Date=01/22/2018, Comments=B