理解Java中的注释 [英] Understanding annotation in Java

问题描述

我试图通过一些在线资料来学习java中的注释。

I was trying to go through some online material to learn annotation in java.

在下面的代码中，我亲爱的Hello world字符串发生了什么传递到这一行： @Test_Target（doTestTarget =Hello World！）？

In the following code, what happened to my dear "Hello world" string which I passed in this line: @Test_Target(doTestTarget="Hello World !")?

@Target(ElementType.METHOD)
public @interface Test_Target {
   public String doTestTarget();
}

上面是定义的注释，下面是它的用法

above is the annotation defined and below is its usage

public class TestAnnotations {
   @Test_Target(doTestTarget="Hello World !")
   private String str;
   public static void main(String arg[]) {
      new TestAnnotations().doTestTarget();
   }
   public void doTestTarget() {
      System.out.printf("Testing Target annotation");
   }
}

当我运行此代码时，它只打印测试目标注释

When I run this code it is only printing Testing Target annotation

请帮助我，我对注释完全不熟悉。

Please help me out, I am completely new to annotation.

推荐答案

注释基本上是可以附加到字段，方法，类等的数据位。

Annotations are basically bits of data you can attach to fields, methods, classes, etc.

在Java中声明注释的语法有点尴尬。它们看起来有点像接口（毕竟它们是用 @interface 声明的），但它们不是真正的接口。我想您可能已经在 TestAnnotations 类中放置了 doTestTarget（）方法，因为您认为您的注释是一个接口，你需要实现它。事实并非如此 - 如果您愿意，可以删除此方法并从代码中调用它，这样做不会给您带来任何问题。

The syntax for declaring annotations in Java is a little awkward. They look a bit like interfaces (they are, after all, declared with @interface), but they aren't really interfaces. I think you might have put the doTestTarget() method in your TestAnnotations class because you thought your annotation was an interface and you needed to implement it. This isn't true - you can delete this method and the call to it from your code if you wish and doing so won't cause you any problems.

此外，您可能没有打算将注释放在字段 str 上。注释仅适用于紧随其后的内容。因此，您的代码无法编译，因为您已将注释应用于字段，但声明您的注释只能应用于方法。将 @Target（ElementType.METHOD）更改为 @Target（ElementType.FIELD）然后编译代码。

Also, you might not have intended to put the annotation on the field str. Annotations apply only to what immediately follows them. As a result, your code doesn't compile, because you've applied your annotation to a field but declared that your annotation can only be applied to methods. Change @Target(ElementType.METHOD) to @Target(ElementType.FIELD) and your code should then compile.

至于字符串 Hello World！会发生什么，它会被写入.class文件并可用于任何读入Java类的工具。但是，它不一定在运行时在JVM中可用。发生这种情况是因为您没有为 @Test_Target 注释指定 @Retention 。 @Retention 的默认值是 RetentionPolicy.CLASS ，这意味着JVM可能无法将它们加载到类文件。 （请参阅 Javadoc for RetentionPolicy enum 。）

As for what happens to the string Hello World !, it gets written to the .class file and is available to any tool that reads in Java classes. However, it wouldn't necessarily be available in the JVM at runtime. This happens because you didn't specify a @Retention for your @Test_Target annotation. The default value for @Retention is RetentionPolicy.CLASS, which means that the JVM might not bother to load them out of the class file. (See the Javadoc for the RetentionPolicy enum.)

我想你想看到一些在运行时从这个注释中读取值的方法。如果是这样，我建议在注释中添加 @Retention（RetentionPolicy.RUNTIME），以确保它在运行时可用。

I imagine you want to see some way of reading the value out of this annotation at runtime. If so, I'd recommend adding @Retention(RetentionPolicy.RUNTIME) to your annotation to make sure it will be available at runtime.

要在运行时访问注释及其中包含的值，您需要使用反射。我已按如下方式重写您的 TestAnnotations 类，以便快速演示：

To access your annotation and the value contained within it at runtime, you need to use reflection. I've rewritten your TestAnnotations class as follows to give a quick demonstration:

import java.lang.reflect.Field;

public class TestAnnotations {

   @Test_Target(doTestTarget="Hello World !")
   private String str;

   public static void main(String[] args) throws Exception {
      // We need to use getDeclaredField here since the field is private.
      Field field = TestAnnotations.class.getDeclaredField("str");
      Test_Target ann = field.getAnnotation(Test_Target.class);
      if (ann != null) {
         System.out.println(ann.doTestTarget());
      }
   }
}

当我运行此代码时，它给了我以下输出：

When I run this code, it gives me the following output:

Hello World !

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