一、定义br/>Java文件叫做Annotation，用@interface表示。
java中提供了四种元注解，用于创建新的注解，分别是：@Retention、@Target、@Document、@Inherited
二、元注解
@Retention 从源代码中可以看出，主要用于提示注解要保留多长时间
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();
    }
    有三种取值：
    RetentionPolicy.SOURCE 将会被编译器抛弃
    RetentionPolicy.CLASS 注解会被编辑器保留在类文件中，但是会被vm抛弃
    RetentionPolicy.RUNTIME 注解会被编辑器保留在类文件中，也会被vm保留，所以可以通过反射读取。
    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 用于提示该注解使用的地方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的取值有下面十种：
    ElementType.TYPE 用于类，接口（包括注解）或者枚举类型
    ElementType.FIELD 用于属性字段包括枚举常量
    ElementType.METHOD 用于方法级别
    ElementType.PARAMETER 用于参数声明
    ElementType.CONSTRUCTOR 用于构造函数声明
    ElementType.LOCAL_VARIABLE 用于局部变量声明
    ElementType.ANNOTATION_TYPE 用于注解类型声明
    ElementType.PACKAGE 用于包声明
    ElementType.TYPE_PARAMETER 用于泛型声明
    ElementType.TYPE_USE 用于任意类型声明
    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 将注解包含在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 允许子类继承父类
  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 {
  }
  三、自定义注解的使用
  创建一个自定义注解
  import java.lang.annotation.*;
  /**
• Created Date: 2019/3/1

• 创建自定义注解
  */
  @Target({ ElementType.METHOD, ElementType.TYPE })br/>@Retention(RetentionPolicy.RUNTIME)
  @Documented
  public @interface TestAnnotation {

  String value();
  }
  通过反射获取注解
  public class Test {
  @TestAnnotation(value = "测试方法")
  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();
  }
  }
  }
  四、web开发中的运用
  在web开发中，权限控制非常重要，所以有些接口会限制必须登录之后才能访问，但是个别接口并没有这种限制。一种方式是把需要过滤的ThinkMarkets代理申请www.kaifx.cn/broker/thinkmarkets.html接口或者方法配置在文件中，每次请求时在拦截器中根据请求的路径与配置文件中的对比过滤。其实还有另外一种方式就是通过注解方式。
  定义一个注解NoLogin
  @Target(ElementType.METHOD)
  @Retention(RetentionPolicy.RUNTIME)br/>@Documented
  public @interface NoLogin {
  }
  标注在方法上
  在拦截器中判断方法上是否有NoLogin注解
  public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object o) throws Exception {
  //支持两种方式过滤 1、注解方式 添加@NoLogin注解
  HandlerMethod handlerMethod= (HandlerMethod) o;
  NoLogin noLogin=handlerMethod.getMethod().getDeclaredAnnotation(NoLogin.class);
  if(null!=noLogin){
  return true;
  }
  }
  五、java内置的注解
  除了上述的四个元注解，java还内置了另外三个注解：
  @Override 它没有任何的属性，不能存储任何其他信息。它只能作用于方法之上，编译结束后将被丢弃。在java编译器编译成字节码的时候，一旦发现某个方法被这个注解标识过，就会匹配父类中是否存在同一方法，如果不存在就回编译失败。br/>@Target(ElementType.METHOD)
  @Retention(RetentionPolicy.SOURCE)
  public @interface Override {
  }
  @Deprecated 弃用的注解br/>@Documented
  @Retention(RetentionPolicy.RUNTIME)
  @Target(value={CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER, TYPE})
  public @interface Deprecated {
  }
  @SuppressWarnings 压制警告，比如某段代码中存在过时的方法，那么在编译过程中，会有warn警告，如果不想出现类似的警告，可在方法上添加这个注解。这个注解有一个value的值，这个value表示需要压制的警告类型。
  @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();
    }
    六、注解的原理
    1、java.lang.annotation.Annotation中有这么一句话：The common interface extended by all annotation types 所有的注解都继承于这个接口。怎么理解呢？其实刚才上面例子中的注解可以理解为：
    public @interface TestAnnotation extends Annotation{
    }
    注解的本质就是一个继承了 Annotation 接口的接口
    为了方便理解和掌握注解，还是以刚才的TestAnnotation注解为例。在idea中配置启动参数，方便查看代理产生的类，参数如下：
    Dsun.misc.ProxyGenerator.saveGeneratedFiles=true
    main方法运行结束后，会在/com/sun/proxy目录下生成一个代理类，反编译之后是这样的：
    代理类proxy1重写了TestAnnotation的所有方法，包括value()和从Annotation继承来的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构造函数中有一个接口InvocationHandler，这个接口的实例化对象又是谁？
    java中有一个专门用于注解类型的代理对象AnnotationInvocationHandler，位于sun.reflect.annotation包中。同样是invoke方法用于处理具体的业务。
    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;
    }
    }
    }
    }