Java註解反射
程序元素上的註解是Java對象。允許訪問其註解的程序元素實現java.lang.reflect.AnnotatedElement接口。以下類實現了AnnotatedElement接口：
AnnotatedElement接口的方法用於訪問以下列出的對象類型的註解。
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
註解類型必須使用運行時的保留策略來保留元註解，以便在運行時訪問它。
示例 - 1
假設有一個Test類，並且想打印它的所有註解。參考以下代碼片段，它將打印Test類的類聲明上的所有註解：
import java.lang.annotation.Annotation;
@SuppressWarnings("unchecked")
@Deprecated
public class Main {
public static void main(String[] argv) {
// Get the class object reference
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);
} }
}
執行上面的代碼得到如下結果 -
Annotation count: 1
@java.lang.Deprecated()
如上面的結果所示，Annotation接口的toString()方法返回註解的字符串表示形式。
實例-2
以下代碼顯示瞭如何獲取指定的註解，請參考以下代碼 -
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
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);
} }
}
上面的代碼生成以下結果。
Version: major=1, minor=2
實例-3
以下代碼顯示瞭如何訪問方法的註解，請參考以下代碼 -
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
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);
}
}
}
}
上面的代碼生成以下結果。
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()
實例-4
以下代碼顯示瞭如何在運行時訪問可重複註解的實例，請參考以下代碼 -
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
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());
} }
}
上面的代碼生成以下結果。
Date=02/01/2018, Comments=A
Date=01/22/2018, Comments=B
Date=02/01/2018, Comments=A
Date=01/22/2018, Comments=B