Understanding Synchronous, Asynchronous Calls and Callbacks in Java

Calling and Callback Mechanisms

In any application system, modules inevitably call each other; the invocation methods can be classified into several types.

1. Synchronous Call

Synchronous call is the most basic and simple form: method a() in class A calls method b() in class B and waits until b() finishes before continuing. It is suitable when b() executes quickly; otherwise the whole flow may be blocked.

2. Asynchronous Call

Asynchronous call solves the blocking problem of synchronous calls. Class A starts a new thread to invoke class B's method b(); the code proceeds immediately, so the execution of a() is never blocked regardless of how long b() runs.

If a() later needs the result of b(), it must listen for the result, e.g., using Future + Callable in Java.

http://www.cnblogs.com/xrq730/p/4872722.html

3. Callback

Callback is a bidirectional invocation method; it can be synchronous or asynchronous. The typical flow is: class A's a() calls class B's b(); after b() finishes, it actively calls A's callback() method.

Class A's a() method calls class B's b() method.

Class B's b() method, after execution, invokes class A's callback() method.

In simple terms, A calls B's method C, and B calls back A's method D, which is the callback method.

Types of Callbacks

Callbacks are divided into synchronous and asynchronous. Using a lottery scenario: buying a ticket and waiting for the result is a synchronous callback (must wait); buying a ticket and doing other things while waiting for the result is an asynchronous callback.

Synchronous Callback

Synchronous callback behaves like a normal call; if the called API takes a long time, the caller must wait, which blocks the flow.

Example

public interface OrderResult {
    /**
     * Order status
     * @param state
     * @return
     */
    String getOrderResult(String state);
}

public class Store {
    @Getter @Setter
    private String name;
    public Store(String name) { this.name = name; }
    /** Callback function: pass the structure to a method we cannot call directly and get the result */
    public String returnOrderGoodsInfo(OrderResult order) {
        String[] s = {"订购中...", "订购失败", "即将发货!", "运输途中...", "已在投递"};
        Random random = new Random();
        int temp = random.nextInt(5);
        String s1 = s[temp];
        return order.getOrderResult(s1);
    }
}

public class SyncBuyer implements OrderResult {
    @Getter @Setter private Store store;
    @Getter @Setter private String buyerName;
    @Getter @Setter private String goodsName;
    public SyncBuyer(Store store, String buyerName, String goodsName) {
        this.store = store; this.buyerName = buyerName; this.goodsName = goodsName;
    }
    public String orderGoods() {
        String goodsState = store.returnOrderGoodsInfo(this);
        System.out.println(goodsState);
        myFeeling();
        return goodsState;
    }
    public void myFeeling() {
        String[] s = {"有点小激动", "很期待!", "希望是个好货!"};
        Random random = new Random();
        int temp = random.nextInt(3);
        System.out.println("我是" + getBuyerName() + ", 我现在的感觉: " + s[temp]);
    }
    @Override
    public String getOrderResult(String state) {
        return "在" + getStore().getName() + "商店订购的" + getGoodsName() + "玩具, 目前的预订状态是: " + state;
    }
}

public class Test2Callback {
    public static void main(String[] args) {
        Store wallMart = new Store("沙中路沃尔玛");
        SyncBuyer syncBuyer = new SyncBuyer(wallMart, "小明", "超能铁扇公主");
        System.out.println(syncBuyer.orderGoods());
    }
}

Asynchronous Callback

The code difference between sync and async callbacks is whether a new thread is created when invoking the third‑party API.

Example

public interface OrderResult {
    String getOrderResult(String state);
}

public class Store {
    @Getter @Setter private String name;
    public Store(String name) { this.name = name; }
    public String returnOrderGoodsInfo(OrderResult order) {
        String[] s = {"订购中...", "订购失败", "即将发货!", "运输途中...", "已在投递"};
        Random random = new Random();
        int temp = random.nextInt(5);
        String s1 = s[temp];
        return order.getOrderResult(s1);
    }
}

public class NoSyncBuyer implements OrderResult {
    @Getter @Setter private Store store;
    @Getter @Setter private String buyerName;
    @Getter @Setter private String goodsName;
    public NoSyncBuyer(Store store, String buyerName, String goodsName) {
        this.store = store; this.buyerName = buyerName; this.goodsName = goodsName;
    }
    public String orderGoods() {
        String goodsState = "--";
        MyRunnable mr = new MyRunnable();
        Thread t = new Thread(mr);
        t.start();
        System.out.println(goodsState);
        goodsState = mr.getResult();
        myFeeling();
        return goodsState;
    }
    public void myFeeling() {
        String[] s = {"有点小激动", "很期待!", "希望是个好货!"};
        Random random = new Random();
        int temp = random.nextInt(3);
        System.out.println("我是" + getBuyerName() + ", 我现在的感觉: " + s[temp]);
    }
    @Override
    public String getOrderResult(String state) {
        return "在" + getStore().getName() + "商店订购的" + getGoodsName() + "玩具, 目前的预订状态是: " + state;
    }
    private class MyRunnable implements Runnable {
        @Getter @Setter private String result;
        @Override
        public void run() {
            try {
                Thread.sleep(10000);
                result = store.returnOrderGoodsInfo(NoSyncBuyer.this);
            } catch (InterruptedException e) {
                // log error
            }
        }
    }
}

public class Test2Callback {
    public static void main(String[] args) {
        Store wallMart = new Store("沙中路沃尔玛");
        SyncBuyer syncBuyer = new SyncBuyer(wallMart, "小明", "超能铁扇公主");
        System.out.println(syncBuyer.orderGoods());
        System.out.println();
        Store lawson = new Store("沙中路罗森便利店");
        NoSyncBuyer noSyncBuyer = new NoSyncBuyer(lawson, "cherry", "变形金刚");
        System.out.println(noSyncBuyer.orderGoods());
    }
}