《Java網路程式設計》第三章執行緒講使用回撥的方法從執行緒中返回資訊給物件。例如，如果有多個物件對執行緒的計算結果感興趣，那麼執行緒可以儲存一個要回調的物件列表。特定的物件可以通過呼叫執行緒類的一個方法把自己新增到這個列表中來完成註冊，表示自己對計算結果很感興趣。如果有多個類的例項對結果感興趣，可以定義一個新的Observer interface （介面），所有這些類都要實現這個新介面。這個Observer interface（介面）將宣告回撥方法。以上是觀察者模式的典型應用場景。

接下來將上面的需求實現。

1.       Observer介面

宣告回撥方法update

interface Observer {
    void update(byte[] digest);
}
 

2.       感興趣的觀察者InstanceCallbackDigestUserInterface01

其中將通知者作為其成員變數

public class InstanceCallbackDigestUserInterface01 implements Observer {

    static Logger logger = LoggerFactory.getLogger(InstanceCallbackDigestUserInterface01.class);

    private String filename;
    private byte[] digest;

    public InstanceCallbackDigestUserInterface01(String filename) {
        this.filename = filename;
    }

    @Override
    public String toString() {
        String result = filename + ": ";
        if (digest != null) {
            result += DatatypeConverter.printHexBinary(digest);
        } else {
            result += "digest not available";
        }
        return result;
    }

    @Override
    public void update(byte[] digest) {
        this.digest = digest;
        //只是顯示摘要，但功能更強的類還可以完成其他操作
        //如將摘要儲存在一個欄位中，用它啟動另一個錢程，或者對它完成進一步的計算。
        logger.info(this.toString());
    }
}
 

3.       感興趣的觀察者InstanceCallbackDigestUserInterface02以及新執行緒任務SaveDatabase

public class InstanceCallbackDigestUserInterface02 implements Observer {

    Logger logger = LoggerFactory.getLogger(InstanceCallbackDigestUserInterface02.class);

    byte[] digest = null;

    @Override
    public void update(byte[] digest) {
        logger.info("interface02 get this digest");
        this.digest = digest;
        ExecutorService exec = Executors.newCachedThreadPool();
        exec.submit(new SaveDatabase(digest));
    }
}

class SaveDatabase implements Runnable {

    Logger logger = LoggerFactory.getLogger(SaveDatabase.class);
    private byte[] digest;

    public SaveDatabase(byte[] digest){
        this.digest = digest;
    }

    @Override
    public void run() {
        String s = DatatypeConverter.printHexBinary(digest);
        logger.info(s);
    }
}
 

4.       通知者InstanceCallbackDigest

儲存一個要回調的物件列表callback。特定的物件可以通過呼叫attach方法把自己新增到這個列表中來完成註冊

public class InstanceCallbackDigest implements Runnable {

    Logger logger = LoggerFactory.getLogger(InstanceCallbackDigest.class);

    private String filename;
    private LinkedHashSet<Observer> callback = new LinkedHashSet<>();

    public InstanceCallbackDigest(String filename) {
        this.filename = filename;
    }

    public void attach(Observer observer) {
        callback.add(observer);
    }

    public void detach(Observer observer) {
        callback.remove(observer);
    }

    @Override
    public void run() {
        try {
            FileInputStream in = new FileInputStream(filename);
            MessageDigest sha = MessageDigest.getInstance("SHA-256");
            DigestInputStream din = new DigestInputStream(in, sha);
            while (din.read() != -1) ;  // read entire file
            din.close();
            byte[] digest = sha.digest();
            for (Observer o:callback
                 ) {
                logger.info("callback update");
                o.update(digest);
            }
        } catch (IOException | NoSuchAlgorithmException ex) {
            System.err.println(ex);
        }
    }
}

5.       Test類

public class Test {
    static Logger logger = LoggerFactory.getLogger(Test.class);

    public void calculateDigest(String filename, LinkedHashSet<Observer> callback) {
        InstanceCallbackDigest cb = new InstanceCallbackDigest(filename);
        //註冊
        for (Observer o : callback
                ) {
            cb.attach(o);
        }
        Thread t = new Thread(cb);
        t.start();
    }

    @org.junit.Test
    public void test () {
        logger.info("main執行緒");
        String[] strs = new String[]{"bw01.txt","pom.xml"};
//        String[] strs = new String[]{"bw01.txt"};
        for (String filename : strs) {
            //註冊
            InstanceCallbackDigestUserInterface01 d = new InstanceCallbackDigestUserInterface01(filename);
            InstanceCallbackDigestUserInterface02 interface02 = new
                    InstanceCallbackDigestUserInterface02();
            LinkedHashSet<Observer> callback = new LinkedHashSet<>();
            callback.add(d);
            callback.add(interface02);

            //啟動了執行緒
            calculateDigest(filename, callback);

            //如何通知主執行緒子執行緒結束?
            try {
                TimeUnit.SECONDS.sleep(3l);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

6.       類圖

7.       執行結果