註解用法

Annotations have a number of uses, among them:

• Information for the compiler — Annotations can be used by the compiler to detect errors or suppress warnings.
• Compile-time and deployment-time processing — Software tools can process annotation information to generate code, XML files, and so forth.
• Runtime processing — Some annotations are available to be examined at runtime.

註解用在哪些地方

Where Annotations Can Be Used

Annotations can be applied to declarations: declarations of classes, fields, methods, and other program elements. When used on a declaration, each annotation often appears, by convention, on its own line.

As of the Java SE 8 release, annotations can also be applied to the use of types. Here are some examples:

• Class instance creation expression:

      new @Interned MyObject();
  

• Type cast:

      myString = (@NonNull String) str;
  

• implements clause:

      class UnmodifiableList<T> implements
          @Readonly List<@Readonly T> { ... }
   
  

• Thrown exception declaration:

      void monitorTemperature() throws
          @Critical TemperatureException { ... }

Java語言用到的註解

@Deprecated @Override @SuppressWarnings @SafeVarargs @FunctionalInterface

適用於其他註解的註解（主要了解@Retention,@Target,@Repeatable）

@Retention @Retention annotation specifies how the marked annotation is stored:

• RetentionPolicy.SOURCE – The marked annotation is retained only in the source level and is ignored by the compiler.
• RetentionPolicy.CLASS – The marked annotation is retained by the compiler at compile time, but is ignored by the Java Virtual Machine (JVM).
• RetentionPolicy.RUNTIME – The marked annotation is retained by the JVM so it can be used by the runtime environment.

@Documented @Documented annotation indicates that whenever the specified annotation is used those elements should be documented using the Javadoc tool. (By default, annotations are not included in Javadoc.) For more information, see theJavadoc tools page.

@Target @Target annotation marks another annotation to restrict what kind of Java elements the annotation can be applied to. A target annotation specifies one of the following element types as its value:

• 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.

@Inherited @Inherited annotation indicates that the annotation type can be inherited from the super class. (This is not true by default.) When the user queries the annotation type and the class has no annotation for this type, the class' superclass is queried for the annotation type. This annotation applies only to class declarations.

@Repeatable @Repeatable annotation, introduced in Java SE 8, indicates that the marked annotation can be applied more than once to the same declaration or type use. For more information, see Repeating Annotations.

型別註解和可插入型別系統（主要了解開源的Check Framework框架）

For example, you want to ensure that a particular variable in your program is never assigned to null; you want to avoid triggering a NullPointerException. You can write a custom plug-in to check for this. You would then modify your code to annotate that particular variable, indicating that it is never assigned to null. The variable declaration might look like this:

@NonNull String str;

When you compile the code, including the NonNull module at the command line, the compiler prints a warning if it detects a potential problem, allowing you to modify the code to avoid the error. After you correct the code to remove all warnings, this particular error will not occur when the program runs.

You can use multiple type-checking modules where each module checks for a different kind of error. In this way, you can build on top of the Java type system, adding specific checks when and where you want them.

With the judicious use of type annotations and the presence of pluggable type checkers, you can write code that is stronger and less prone to error.

In many cases, you do not have to write your own type checking modules. There are third parties who have done the work for you. For example, you might want to take advantage of the Checker Framework created by the University of Washington. This framework includes a NonNull module, as well as a regular expression module, and a mutex lock module. For more information, see the Checker Framework.