使用 ThreadLocal 实现上下文

为了避免使用方法直接显示的参数传递，我们可以采用 ThreadLocal 实现上下文来存储和获取参数，假设我们现在需要实现提现上下文 WithdrawContext，首先定义一个上下文类 WithdrawContext，并定义对应上下文管理器 WithdrawManager：

@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class WithdrawContext {
    // eg
    private String withdrawId;
}

public class WithdrawContextManager {

    private static final ThreadLocal<WithdrawContext> THREAD_LOCAL = new InheritableThreadLocal<>();

    /**
     * init Context
     *
     * @param context WithdrawContext
     */
    public static void setContext(WithdrawContext context) {
        THREAD_LOCAL.set(context);
    }

    /**
     * get current thread context
     *
     * @return WithdrawContext
     */
    public static WithdrawContext getContext() {
        return THREAD_LOCAL.get();
    }

    /**
     * clear conext
     */
    public static void removeContext() {
        THREAD_LOCAL.remove();
    }
}

为了简化开发，我们可以采用切面来初始化和销毁上下文，首先我们定义如下注解，并采用 AOP 切面来针对声明有对应注解的方法进行切面处理：

// 该注解可被声明在构造器方法和普通方法上
@Target({ElementType.CONSTRUCTOR, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface WithdrawContextHolder {
}

@Component
@Aspect
public class WithdrawContextAspect {

    @Pointcut(value = "@annotation(com.shoto.threadlocal.WithdrawContextHolder)")
    public void pointCut() {

    }

    @Around("pointCut()")
    public Object invokeAround(ProceedingJoinPoint joinPoint) throws Throwable {
        try {
            WithdrawContextManager.removeContext();
            WithdrawContextManager.setContext(new WithdrawContext());
            return joinPoint.proceed();
        } finally {
            // 销毁方法是必须的，防止 OOM
            WithdrawContextManager.removeContext();
        }
    }
}

至此，我们便可以在 Service 方法上进行使用，

@Service
public class WithdrawServiceImpl implements WithdrawService {

    @Override
    @WithdrawContextHolder // 注解声明
    public String getWithdrawId() {
        WithdrawContext withdrawContext = WithdrawContextManager.getContext();
        withdrawContext.setWithdrawId(UUID.randomUUID().toString());
        // 这里我们无须在 queryWithdraw 上传递 WithdrawContext 参数即可获取上下文参数信息
        return queryWithdraw();
    }

    private String queryWithdraw() {
        WithdrawContext withdrawContext = WithdrawContextManager.getContext();
        return withdrawContext.getWithdrawId();
    }
}