明天来讲讲JAVA种罕用的几种注解，比拟容易应用，也简略

@Override查看笼罩办法是否正确
@Deprecated将这个办法置为不举荐应用的办法
@SupperessWarnings将某种正告提醒疏忽掉，能够放在类后面讲将整个类的这种正告疏忽，也能够放在办法头上，只疏忽这个办法的正告

public class Doer {    @Deprecated    void doItThisWay(){ }    void doItThisNewWay(){ }}@SuppressWarnings("deprecation")public class MyWorker {    @SuppressWarnings("deprecation")    void doSomeWork(){        Doer d = new Doer();        d.doItThisWay();    }    @SuppressWarnings("deprecation")    void doDoubleWork(){        Doer d1 = new Doer();        Doer d2 = new Doer();        d1.doItThisWay();        d2.doItThisWay();    }}

自定义注解，通过@interface的形式定义，通过@新注解名字来应用注解
其中Target指定该新定义的注解能够应用在哪个中央@Target(ElementType.TYPE)，@Target({ElementType.TYPE,ElementType.METHOD})等示意
另外Retention示意能够在何时能够获取到该新注解，RUNTIME示意在运行时能够获取到（想要通过反射获取到必须应用这种RUNTIME）
定义的变量能够设置为default值

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)public @interface WorkHandler {    boolean useThreadPool() default true;}

应用新注解，当下面的定义default时true时候，上面如果是（useThreadPool = true）则能够去掉，除非设置为false必填写
这样通过注解能够判断是否应用线程池

@WorkHandler(useThreadPool = true)public class AccountWorker implements Runnable, TaskWorker {    BankAccount ba;    public void setTarget(Object target) {        if(BankAccount.class.isInstance(target))            ba = (BankAccount) target;        else            System.out.println("Not BankAccount");    }    public void doWork(){        Thread t = new Thread(HighVolumeAccount.class.isInstance(ba) ? (HighVolumeAccount)ba: this);        t.start();    }    @Override    public void run() {        char txType = 'w';        int amt = 200;        if(txType =='w')            ba.withdrawal(amt);        else            ba.deposit(amt);    }}

执行办法：

com.example.demo.pluralsight.runtimetype.BankAccount acct2 = new com.example.demo.pluralsight.runtimetype.BankAccount("2");startWork("com.example.demo.pluralsight.metadatawithannotations.AccountWorker",acct2);Thread.sleep(100);System.out.println("TaskAccountWorker:"+acct2.getBalance());private static void startWork(String workerTypeName, Object workerTarget){    try{        Class<?> workerType = Class.forName(workerTypeName);        AccountWorker worker= (AccountWorker)workerType.newInstance();        worker.setTarget(workerTarget);        ExecutorService pool = Executors.newFixedThreadPool(3);        WorkHandler wh = workerType.getAnnotation(WorkHandler.class);        if(wh == null)            throw new Exception("WorkHandler is null");        if(wh.useThreadPool())            pool.submit(new Runnable() {                @Override                public void run() {                    worker.doWork();                }            });        else            worker.doWork();        pool.shutdown();    }catch (Exception e){        e.printStackTrace();    }}

还能够简写，间接定义新注解的变量名称为value，这样在类的后面写入的注解就能够间接写成@WorkHandler(false)

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)public @interface WorkHandler {    boolean value() default true;}

自定义注解的变量类型能够为，Primitive type，String，Enum，Annotation，Class<?>，同时也能够是这些类型的数组。
传入class的形式，这样又能够节俭一个传入参数

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)public @interface ProcessedBy {    Class<?> value();}@ProcessedBy(AccountWorkerProcessBy.class)public class BankAccount {    private String id;    private int balance = 0;    public BankAccount(String id){        this.id = id;    }    public BankAccount(String id,int startBalance){        this.id = id;        balance = startBalance;    }    public String getId(){        return id;    }    public synchronized int getBalance(){        return balance;    }    public synchronized void deposit(int amount){        balance += amount;    }    public synchronized void withdrawal(int amount){        balance -= amount;    }}public class AccountWorkerProcessBy implements Runnable {    BankAccount ba;    public void setTarget(Object target) {        if(BankAccount.class.isInstance(target))            ba = (BankAccount) target;        else            System.out.println("Not BankAccount");    }    public void doWork(){        Thread t = new Thread(this);        t.start();    }    @Override    public void run() {        char txType = 'w';        int amt = 200;        if(txType =='w')            ba.withdrawal(amt);        else            ba.deposit(amt);    }}

执行办法：

BankAccount acct3 = new BankAccount("3");startWorkSelfContained(acct3);System.out.println("startWorkSelfContained:"+acct3.getBalance());private static void startWorkSelfContained(Object workerTarget) throws Exception{    Class<?> targetType = workerTarget.getClass();    ProcessedBy pb = targetType.getAnnotation(ProcessedBy.class);    Class<?> workerType = pb.value();    AccountWorkerProcessBy worker = (AccountWorkerProcessBy) workerType.newInstance();    worker.setTarget(workerTarget);    worker.doWork();}输入startWorkSelfContained:-200