context上下文模式

• 有时候会一条线程会分步骤进行，第二个步骤依赖于第一个步骤运行的结果。在多线程的情况下很容易出现线程安全的问题。这其中需要一个线程安全的中间上下问来帮助。
• 优化：借助ThreadLocal(有点像一个以线程为key值的map)来构建上下文。他相当于是每一个线程的自己的保险箱。
• Context 上下文具体类

public class Context {

    private String name;
    private String cardId;

    public void setName(String name) {
        this.name = name;
    }

    public String getName() {
        return this.name;
    }

    public void setCardId(String cardId) {
        this.cardId = cardId;
    }

    public String getCardId() {
        return cardId;
    }
}

• ActionContext(关键，借助ThreadLocal,把Context放入，自己是单例的)

public final class ActionContext {

    private static final ThreadLocal<Context> threadLocal = new ThreadLocal<Context>() {
        @Override
        protected Context initialValue() {
            return new Context();
        }
    };

    private static class ContextHolder {
        private final static ActionContext actionContext = new ActionContext();
    }

    public static ActionContext getActionContext() {
        return ContextHolder.actionContext;
    }

    public Context getContext() {
        return threadLocal.get();
    }

    private ActionContext(){

    }
}

• 模拟第一个步骤,从数据库获取数据，此时与context发生了交互

public class QueryFromDBAction {

    public void execute() {
        try {
            Thread.sleep(1000L);
            String name = "Alex " + Thread.currentThread().getName();
            ActionContext.getActionContext().getContext().setName(name);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

• 第二个步骤依赖于第一个步骤，模拟从http获取数据

public class QueryFromHttpAction {

    public void execute() {
        Context context = ActionContext.getActionContext().getContext();
        String name = context.getName();
        String cardId = getCardId(name);
        context.setCardId(cardId);
    }

    private String getCardId(String name) {
        try {
            Thread.sleep(1000L);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        return "435467523543" + Thread.currentThread().getId();
    }
}

• 线程真正运行的Runnable(有了上下文，即使是在执行后再创建上下文对象，无需交互，也能获取想要的数值)

public class ExecutionTask implements Runnable {

    private QueryFromDBAction queryAction = new QueryFromDBAction();

    private QueryFromHttpAction httpAction = new QueryFromHttpAction();

    @Override
    public void run() {

        queryAction.execute();
        System.out.println("The name query successful");
        httpAction.execute();
        System.out.println("The card id query successful");

        Context context = ActionContext.getActionContext().getContext();
        System.out.println("The Name is " + context.getName() + " and CardId " + context.getCardId());
    }
}

• 运行类，创建线程

public class ContextTest {
    public static void main(String[] args) {
        IntStream.range(1, 5)
                .forEach(i ->
                        new Thread(new ExecutionTask()).start()
                );
    }
}