註解
註解的作用:
- 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.
註解格式,使用@
:
@Entity
帶key-value:
@Author(
name = "Benjamin Franklin",
date = "3/27/2003"
)
class MyClass { ... }
只有1個key時可以省略key:
@SuppressWarnings(value = "unchecked")
void myMethod() { ... }
@SuppressWarnings("unchecked")
void myMethod() { ... }
同時使用多個註解:
@Author(name = "Jane Doe")
@EBook
class MyClass { ... }
The annotation type can be one of the types that are defined in the java.lang
or java.lang.annotation
packages of the Java SE API.
哪裏能用註解
① Annotations can be applied to declarations: declarations of classes, fields, methods, and other program elements.
② Java SE 8,annotations can also be applied to the use of types:
-
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 { ... }
自定義註解
使用@interface
:
@interface ClassPreamble {
String author();
String date();
int currentRevision() default 1;
String lastModified() default "N/A";
String lastModifiedBy() default "N/A";
// Note use of array
String[] reviewers();
}
註解其實也是一種接口,只是要使用@
來聲明。
使用:
@ClassPreamble (
author = "John Doe",
date = "3/17/2002",
currentRevision = 6,
lastModified = "4/12/2004",
lastModifiedBy = "Jane Doe",
// Note array notation
reviewers = {"Alice", "Bob", "Cindy"}
)
public class Generation3List extends Generation2List {
// class code goes here
}
Java預置註解
@Deprecated
// Javadoc comment follows
/**
* @deprecated
* explanation of why it was deprecated
*/
@Deprecated
static void deprecatedMethod() { }
@Override
// mark method as a superclass method
// that has been overridden
@Override
int overriddenMethod() { }
@SuppressWarnings
// use a deprecated method and tell
// compiler not to generate a warning
@SuppressWarnings("deprecation")
void useDeprecatedMethod() {
// deprecation warning
// - suppressed
objectOne.deprecatedMethod();
}
@SafeVarargs
@FunctionalInterface
註解上的註解:Annotations that apply to other annotations are called meta-annotations. There are several meta-annotation types defined in java.lang.annotation
.
@Retention
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
@Target
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.MODULE
can be applied to a module declaration.ElementType.PACKAGE
can be applied to a package declaration.ElementType.PARAMETER
can be applied to the parameters of a method.ElementType.RECORD_COMPONENT
can be applied to the component of a record.ElementType.TYPE
can be applied to the declaration of a class, an abtract class, an interface, an annotation interface, an enumeration, or a record declaration.ElementType.TYPE_PARAMETER
can be applied on the parameters of a type.ElementType.TYPE_USE
can be applied where a type is used, for instance on the declaration of a field.
@Inherited
@Repeatable
重複註解也是允許的:
@Schedule(dayOfMonth="last")
@Schedule(dayOfWeek="Fri", hour="23")
public void doPeriodicCleanup() { ... }
重複註解的定義要用到@Repeatable:
@Repeatable(Schedules.class)
public @interface Schedule {
String dayOfMonth() default "first";
String dayOfWeek() default "Mon";
int hour() default 12;
}
並且,The containing annotation type must have a value
element with an array type:
public @interface Schedules {
Schedule[] value();
}
the containing annotation type is @Schedules
, so repeating @Schedule
annotations is stored in an @Schedules
annotation.
Schedule[] value()
是一個沒有參數、返回類型爲Schedule[]
的方法的聲明,它是一個抽象方法。事實上,這個方法聲明是用來定義註解的屬性的,與普通方法不同的是,它沒有方法體,只有方法聲明,而方法的具體實現則由使用該註解的代碼來完成。在使用該註解時,也可以通過指定該屬性的值來進行賦值操作,例如:
@Schedules({
@Schedule(dayOfMonth="last"),
@Schedule(dayOfWeek="Fri", hour="23")
})
public class MyScheduledTask {
// ...
}
在上述代碼中,我們使用了@Schedules註解,並且指定了它的value屬性,也就是給Schedule[] value()
方法賦上相應的值。注意到,該屬性的值是一個註解數組,因此需要使用大括號{}
將多個註解組合起來。
參考資料:
Annotations https://dev.java/learn/annotations/