大家好，又见面了，我是你们的朋友全栈君。

一、定义

1. Java文件叫做Annotation，用@interface表示。
2. 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

1. @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 @Retention
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    /**
     * Returns the retention policy.
     * @return the retention policy
     */
    RetentionPolicy value();
}

有三种取值：

1. RetentionPolicy.SOURCE 将会被编译器抛弃

2. RetentionPolicy.CLASS 注解会被编辑器保留在类文件中，但是会被vm抛弃

3. 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
}

2. @Target 用于提示该注解使用的地方

@Documented
@Retention(RetentionPolicy.RUNTIME)
@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：

1. ElementType.TYPE 用于类，接口（包括注解）或者枚举类型

2. ElementType.FIELD 用于属性字段包括枚举常量

3. ElementType.METHOD 用于方法级别

4. ElementType.PARAMETER 用于参数声明

5. ElementType.CONSTRUCTOR 用于构造函数声明

6. ElementType.LOCAL_VARIABLE 用于局部变量声明

7. ElementType.ANNOTATION_TYPE 用于注解类型声明

8. ElementType.PACKAGE 用于包声明

9. ElementType.TYPE_PARAMETER 用于泛型声明

10. 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
}

3. @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 1.5
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Documented {
}

4. @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 @Inherited
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Inherited {
}

三、自定义注解的使用

1. 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 })
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface TestAnnotation {
    
    String value();
}

2. 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) {
            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。

1. 定义一个注解NoLogin

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface NoLogin {
}

2. 标注在方法上
3. 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：

1. @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。

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}

2. @Deprecated 弃用的注解

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(value={CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER, TYPE})
public @interface Deprecated {
}

3. @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})
@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：

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;
                }
            }
        }
    }

var2是具体被调用的方法实例。var4获取方法的名称，然后判断是否equals方法，接着用swtich判断是否来自于annotation中的其他三个方法，toString、hashCode、annotationType。如果是这三个方法，则给var7附上特定的值，并且AnnotationInvocationHandler 实例中已经预定义好了这些方法的实现，直接调用即可。

假如var7没有匹配到上面的那四个方法，则会走下面的逻辑：这里有一个 memberValues，它是一个 Map 键值对，键是我们注解属性名称，值就是该属性当初被赋上的值。
如下图所示，在编译时候会对memberValues赋值，把我们定义好的value字段作为key值放入map中。

在具体执行时，会重新对memberValues赋值，如图所示：

这样就能从注解map中获取我们需要的值。

最后总结一下注解的工作原理：

首先，我们通过键值对的形式可以为注解属性赋值，像这样：@TestAnnotation(value = “测试方法”)

接着，你用注解修饰某个方法或者属性、类等，编译器将在编译期扫描每个注解，会检查注解是否允许作用在当前位置，如果允许，最后会将注解信息写入元素的属性表。

然后在进行反射的时候，jvm会把所有生命周期在 RUNTIME 的注解取出来放到一个 map 中，并创建一个 AnnotationInvocationHandler 实例，把这个 map 传递给它。虚拟机通过JDK 动态代理机制生成一个目标注解的代理类，并初始化好处理器。

那么这样，一个注解的实例就创建出来了，它本质上就是一个代理类。归纳后就是通过方法名返回注解属性值。