java自定義註解的使用

1、定義br/>Java文件叫作Annotation，用@interface表示。
java中提供了四種元註解，用於建立新的註解，分別是：@Retention、@Target、@Document、@Inherited
2、元註解
@Retention 從源代碼中能夠看出，主要用於提示註解要保留多長時間
package java.lang.annotation;
/**

• Indicates how long annotations with the annotated type are to
• be retained. If no Retention annotation is present on
• an annotation type declaration, the retention policy defaults to
• {@code RetentionPolicy.CLASS}.
• <p>A Retention meta-annotation has effect only if the
• meta-annotated type is used directly for annotation. It has no
• effect if the meta-annotated type is used as a member type in
• another annotation type.
• @author Joshua Bloch
• @since 1.5
• @jls 9.6.3.2 @Retentionbr/>*/
  @Documented
  @Retention(RetentionPolicy.RUNTIME)br/>@Target(ElementType.ANNOTATION_TYPE)
  public @interface Retention {
  /**
  • Returns the retention policy.
  • @return the retention policy
    */
    RetentionPolicy value();
    }
    有三種取值：
    RetentionPolicy.SOURCE 將會被編譯器拋棄
    RetentionPolicy.CLASS 註解會被編輯器保留在類文件中，可是會被vm拋棄
    RetentionPolicy.RUNTIME 註解會被編輯器保留在類文件中，也會被vm保留，因此能夠經過反射讀取。
    package java.lang.annotation;
    /**
• Annotation retention policy. The constants of this enumerated type
• describe the various policies for retaining annotations. They are used
• in conjunction with the {@link Retention} meta-annotation type to specify
• how long annotations are to be retained.
• @author Joshua Bloch
• @since 1.5
  */
  public enum RetentionPolicy {
  /**
  • Annotations are to be discarded by the compiler.
    */
    SOURCE,
    /**
  • Annotations are to be recorded in the class file by the compiler
  • but need not be retained by the VM at run time. This is the default
  • behavior.
    */
    CLASS,
    /**
  • Annotations are to be recorded in the class file by the compiler and
  • retained by the VM at run time, so they may be read reflectively.
  • @see java.lang.reflect.AnnotatedElement
    */
    RUNTIME
    }
    @Target 用於提示該註解使用的地方br/>@Documented
    @Retention(RetentionPolicy.RUNTIME)br/>@Target(ElementType.ANNOTATION_TYPE)
    public @interface Target {
    /**
  • Returns an array of the kinds of elements an annotation type
  • can be applied to.
  • @return an array of the kinds of elements an annotation type
  • can be applied to
    */
    ElementType[] value();
    }
    其中ElementType的取值有下面十種：
    ElementType.TYPE 用於類，接口（包括註解）或者枚舉類型
    ElementType.FIELD 用於屬性字段包括枚舉常量
    ElementType.METHOD 用於方法級別
    ElementType.PARAMETER 用於參數聲明
    ElementType.CONSTRUCTOR 用於構造函數聲明
    ElementType.LOCAL_VARIABLE 用於局部變量聲明
    ElementType.ANNOTATION_TYPE 用於註解類型聲明
    ElementType.PACKAGE 用於包聲明
    ElementType.TYPE_PARAMETER 用於泛型聲明
    ElementType.TYPE_USE 用於任意類型聲明
    public enum ElementType {
    / Class, interface (including annotation type), or enum declaration */
    TYPE,
    /* Field declaration (includes enum constants) /
    FIELD,
    / Method declaration */
    METHOD,
    / Formal parameter declaration */
    PARAMETER,
    /* Constructor declaration /
    CONSTRUCTOR,
    / Local variable declaration */
    LOCAL_VARIABLE,
    / Annotation type declaration */
    ANNOTATION_TYPE,
    /* Package declaration /
    PACKAGE,
    /
  • Type parameter declaration
  • @since 1.8
    */
    TYPE_PARAMETER,
    /**
  • Use of a type
  • @since 1.8
    */
    TYPE_USE
    }
    @Documented 將註解包含在Javadoc中
    package java.lang.annotation;
    /**
• Indicates that annotations with a type are to be documented by javadoc
• and similar tools by default. This type should be used to annotate the
• declarations of types whose annotations affect the use of annotated
• elements by their clients. If a type declaration is annotated with
• Documented, its annotations become part of the public API
• of the annotated elements.
• @author Joshua Bloch
• @since 1.5br/>*/
  @Documented
  @Retention(RetentionPolicy.RUNTIME)br/>@Target(ElementType.ANNOTATION_TYPE)
  public @interface Documented {
  }
  @Inherited 容許子類繼承父類
  package java.lang.annotation;
  /**
• Indicates that an annotation type is automatically inherited. If
• an Inherited meta-annotation is present on an annotation type
• declaration, and the user queries the annotation type on a class
• declaration, and the class declaration has no annotation for this type,
• then the class's superclass will automatically be queried for the
• annotation type. This process will be repeated until an annotation for this
• type is found, or the top of the class hierarchy (Object)
• is reached. If no superclass has an annotation for this type, then
• the query will indicate that the class in question has no such annotation.
• <p>Note that this meta-annotation type has no effect if the annotated
• type is used to annotate anything other than a class. Note also
• that this meta-annotation only causes annotations to be inherited
• from superclasses; annotations on implemented interfaces have no
• effect.
• @author Joshua Bloch
• @since 1.5
• @jls 9.6.3.3 @Inheritedbr/>*/
  @Documented
  @Retention(RetentionPolicy.RUNTIME)br/>@Target(ElementType.ANNOTATION_TYPE)
  public @interface Inherited {
  }
  3、自定義註解的使用
  建立一個自定義註解
  import java.lang.annotation.*;
  /**
• Created Date: 2019/3/1

• 建立自定義註解
  */
  @Target({ ElementType.METHOD, ElementType.TYPE })br/>@Retention(RetentionPolicy.RUNTIME)
  @Documented
  public @interface TestAnnotation {

  String value();
  }
  經過反射獲取註解
  public class Test {
  @TestAnnotation(value = "測試方法")
  public static void main(String args[]){
  try {
  Class c=Test.class;
  Method[] methods=c.getDeclaredMethods();
  for(Method method:methods){
  Annotation[] annotations=method.getDeclaredAnnotations();
  for(Annotation annotation:annotations){
  TestAnnotation testAnnotation= (TestAnnotation) annotation;
  System.out.println(testAnnotation.value());
  }
  }
  } catch (ClassNotFoundException e) {br/>e.printStackTrace();
  }
  }
  }
  4、web開發中的運用
  在web開發中，權限控制很是重要，因此有些接口會限制必須登陸以後才能訪問，可是個別接口並無這種限制。一種方式是把須要過濾的ThinkMarkets代理申請www.kaifx.cn/broker/thinkmarkets.html接口或者方法配置在文件中，每次請求時在攔截器中根據請求的路徑與配置文件中的對比過濾。其實還有另一種方式就是經過註解方式。
  定義一個註解NoLogin
  @Target(ElementType.METHOD)
  @Retention(RetentionPolicy.RUNTIME)br/>@Documented
  public @interface NoLogin {
  }
  標註在方法上
  在攔截器中判斷方法上是否有NoLogin註解
  public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object o) throws Exception {
  //支持兩種方式過濾 一、註解方式 添加@NoLogin註解
  HandlerMethod handlerMethod= (HandlerMethod) o;
  NoLogin noLogin=handlerMethod.getMethod().getDeclaredAnnotation(NoLogin.class);
  if(null!=noLogin){
  return true;
  }
  }
  5、java內置的註解
  除了上述的四個元註解，java還內置了另外三個註解：
  @Override 它沒有任何的屬性，不能存儲任何其餘信息。它只能做用於方法之上，編譯結束後將被丟棄。在java編譯器編譯成字節碼的時候，一旦發現某個方法被這個註解標識過，就會匹配父類中是否存在同一方法，若是不存在就回編譯失敗。br/>@Target(ElementType.METHOD)
  @Retention(RetentionPolicy.SOURCE)
  public @interface Override {
  }
  @Deprecated 棄用的註解br/>@Documented
  @Retention(RetentionPolicy.RUNTIME)
  @Target(value={CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER, TYPE})
  public @interface Deprecated {
  }
  @SuppressWarnings 壓制警告，好比某段代碼中存在過期的方法，那麼在編譯過程當中，會有warn警告，若是不想出現相似的警告，可在方法上添加這個註解。這個註解有一個value的值，這個value表示須要壓制的警告類型。
  @Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})br/>@Retention(RetentionPolicy.SOURCE)
  public @interface SuppressWarnings {
  /**

  • The set of warnings that are to be suppressed by the compiler in the
  • annotated element. Duplicate names are permitted. The second and
  • successive occurrences of a name are ignored. The presence of
  • unrecognized warning names is <i>not</i> an error: Compilers must
  • ignore any warning names they do not recognize. They are, however,
  • free to emit a warning if an annotation contains an unrecognized
  • warning name.
  • <p> The string {@code "unchecked"} is used to suppress
  • unchecked warnings. Compiler vendors should document the
  • additional warning names they support in conjunction with this
  • annotation type. They are encouraged to cooperate to ensure
  • that the same names work across multiple compilers.
  • @return the set of warnings to be suppressed*/String[] value();}6、註解的原理一、java.lang.annotation.Annotation中有這麼一句話：The common interface extended by all annotation types 全部的註解都繼承於這個接口。怎麼理解呢？其實剛纔上面例子中的註解能夠理解爲：public @interface TestAnnotation extends Annotation{}註解的本質就是一個繼承了 Annotation 接口的接口爲了方便理解和掌握註解，仍是以剛纔的TestAnnotation註解爲例。在idea中配置啓動參數，方便查看代理產生的類，參數以下：Dsun.misc.ProxyGenerator.saveGeneratedFiles=truemain方法運行結束後，會在/com/sun/proxy目錄下生成一個代理類，反編譯以後是這樣的：代理類proxy1重寫了TestAnnotation的全部方法，包括value()和從Annotation繼承來的equals()、hashCode()等方法。package com.sun.proxy;import java.lang.reflect.InvocationHandler;import java.lang.reflect.Method;import java.lang.reflect.Proxy;import java.lang.reflect.UndeclaredThrowableException;import org.fy.annotation.TestAnnotation;public final class $Proxy1 extends Proxy implements TestAnnotation {private static Method m1;private static Method m2;private static Method m4;private static Method m0;private static Method m3;public $Proxy1(InvocationHandler var1) throws {super(var1);}public final boolean equals(Object var1) throws {try {return (Boolean)super.h.invoke(this, m1, new Object[]{var1});} catch (RuntimeException | Error var3) {throw var3;} catch (Throwable var4) {throw new UndeclaredThrowableException(var4);}}public final String toString() throws {try {return (String)super.h.invoke(this, m2, (Object[])null);} catch (RuntimeException | Error var2) {throw var2;} catch (Throwable var3) {throw new UndeclaredThrowableException(var3);}}public final Class annotationType() throws {try {return (Class)super.h.invoke(this, m4, (Object[])null);} catch (RuntimeException | Error var2) {throw var2;} catch (Throwable var3) {throw new UndeclaredThrowableException(var3);}}public final int hashCode() throws {try {return (Integer)super.h.invoke(this, m0, (Object[])null);} catch (RuntimeException | Error var2) {throw var2;} catch (Throwable var3) {throw new UndeclaredThrowableException(var3);}}public final String value() throws {try {return (String)super.h.invoke(this, m3, (Object[])null);} catch (RuntimeException | Error var2) {throw var2;} catch (Throwable var3) {throw new UndeclaredThrowableException(var3);}}static {try {m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));m2 = Class.forName("java.lang.Object").getMethod("toString");m4 = Class.forName("org.fy.annotation.TestAnnotation").getMethod("annotationType");m0 = Class.forName("java.lang.Object").getMethod("hashCode");m3 = Class.forName("org.fy.annotation.TestAnnotation").getMethod("value");} catch (NoSuchMethodException var2) {throw new NoSuchMethodError(var2.getMessage());} catch (ClassNotFoundException var3) {throw new NoClassDefFoundError(var3.getMessage());}}}在看proxy1構造函數中有一個接口InvocationHandler，這個接口的實例化對象又是誰？java中有一個專門用於註解類型的代理對象AnnotationInvocationHandler，位於sun.reflect.annotation包中。一樣是invoke方法用於處理具體的業務。public Object invoke(Object var1, Method var2, Object[] var3) {String var4 = var2.getName();Class[] var5 = var2.getParameterTypes();if (var4.equals("equals") && var5.length == 1 && var5[0] == Object.class) {return this.equalsImpl(var3[0]);} else if (var5.length != 0) {throw new AssertionError("Too many parameters for an annotation method");} else {byte var7 = -1;switch(var4.hashCode()) {case -1776922004:if (var4.equals("toString")) {var7 = 0;}break;case 147696667:if (var4.equals("hashCode")) {var7 = 1;}break;case 1444986633:if (var4.equals("annotationType")) {var7 = 2;}}switch(var7) {case 0:return this.toStringImpl();case 1:return this.hashCodeImpl();case 2:return this.type;default:Object var6 = this.memberValues.get(var4);if (var6 == null) {throw new IncompleteAnnotationException(this.type, var4);} else if (var6 instanceof ExceptionProxy) {throw ((ExceptionProxy)var6).generateException();} else {if (var6.getClass().isArray() && Array.getLength(var6) != 0) {var6 = this.cloneArray(var6);}return var6;}}}}