Annotations 注解

小结：

1）

a particular variable in your program is never assigned to null; you want to avoid triggering a NullPointerException

@Autowired

Lesson: Annotations (The Java™ Tutorials > Learning the Java Language) https://docs.oracle.com/javase/tutorial/java/annotations/

Lesson: Annotations

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.

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.

This lesson explains where annotations can be used, how to apply annotations, what predefined annotation types are available in the Java Platform, Standard Edition (Java SE API), how type annotations can be used in conjunction with pluggable type systems to write code with stronger type checking, and how to implement repeating annotations.

Answers to Questions and Exercises: Annotations (The Java™ Tutorials > Learning the Java Language > Annotations) https://docs.oracle.com/javase/tutorial/java/annotations/QandE/answers.html

Answers to Questions and Exercises: Annotations

Questions

Question: What is wrong with the following interface:

public interface House {
    @Deprecated
    public void open();
    public void openFrontDoor();
    public void openBackDoor();
}

Answer The documentation should reflect why open is deprecated and what to use instead. For example:

public interface House { 
    /**
     * @deprecated use of open 
     * is discouraged, use
     * openFrontDoor or 
     * openBackDoor instead.
     */
    @Deprecated
    public void open(); 
    public void openFrontDoor();
    public void openBackDoor();
}

Question: Consider this implementation of the House interface, shown in Question 1.

public class MyHouse implements House {
    public void open() {}
    public void openFrontDoor() {}
    public void openBackDoor() {}
}

If you compile this program, the compiler produces a warning because open was deprecated (in the interface). What can you do to get rid of that warning?

Answer: You can deprecate the implementation of open:

public class MyHouse implements House { 
    // The documentation is 
    // inherited from the interface.
    @Deprecated
    public void open() {} 
    public void openFrontDoor() {}
    public void openBackDoor() {}
}

Alternatively, you can suppress the warning:

public class MyHouse implements House { 
    @SuppressWarnings("deprecation")
    public void open() {} 
    public void openFrontDoor() {}
    public void openBackDoor() {}
}

Will the following code compile without error? Why or why not?

public @interface Meal { ... }

@Meal("breakfast", mainDish="cereal")
@Meal("lunch", mainDish="pizza")
@Meal("dinner", mainDish="salad")
public void evaluateDiet() { ... }

Answer: The code fails to compile. Before JDK 8, repeatable annotations are not supported. As of JDK 8, the code fails to compile because the Meal annotation type was not defined to be repeatable. It can be fixed by adding the @Repeatable meta-annotation and defining a container annotation type:

public class AnnotationTest {

    public @interface MealContainer {
        Meal[] value();
    }

    @java.lang.annotation.Repeatable(MealContainer.class)
    public @interface Meal {
        String value();
        String mainDish();
    }

    @Meal(value="breakfast", mainDish="cereal")
    @Meal(value="lunch", mainDish="pizza")
    @Meal(value="dinner", mainDish="salad")
    public void evaluateDiet() { }
}

Exercises

Exercise: Define an annotation type for an enhancement request with elements id, synopsis, engineer, and date. Specify the default value as unassigned for engineer and unknown for date.

Answer:

/**
 * Describes the Request-for-Enhancement (RFE) annotation type.
 */
public @interface RequestForEnhancement {
    int id();
    String synopsis();
    String engineer() default "[unassigned]";
    String date() default "[unknown]";
}

Type Annotations and Pluggable Type Systems

Before the Java SE 8 release, annotations could only be applied to declarations. As of the Java SE 8 release, annotations can also be applied to any type use. This means that annotations can be used anywhere you use a type. A few examples of where types are used are class instance creation expressions (new), casts, implements clauses, and throws clauses. This form of annotation is called a type annotation and several examples are provided in Annotations Basics.

Type annotations were created to support improved analysis of Java programs way of ensuring stronger type checking. The Java SE 8 release does not provide a type checking framework, but it allows you to write (or download) a type checking framework that is implemented as one or more pluggable modules that are used in conjunction with the Java compiler.

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.

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