java中安全的多线程文件创建

发布于 2024-12-21 02:41:48 字数 993 浏览 3 评论 0原文

我有一个 web 应用程序，有时需要从 url 下载一些字节并将其打包并发回给请求者。下载的字节会存储一段时间，以便在需要下载相同的 URL 时可以重复使用它们。我试图找出如何最好地防止线程同时下载相同的 url（如果请求同时传入）。我正在考虑创建一个如下所示的类，以防止同时下载相同的网址。如果一个 url 无法被锁定，那么它要么等到它不再被锁定，然后尝试下载它，只要它在解锁后不存在。

public class URLDownloader
{
    HashMap<String,String> activeThreads = new HashMap<String,String>();

    public synchronized void lockURL(String url, String threadID) throws UnableToLockURLException
    {
        if(!activeThreads.containsKey(url))
            activeThreads.put(url, threadID)
        else
            throw UnableToLockURLException()
    }

    public synchonized void unlockURL(String url, String threadID)
    {
        //need to check to make sure its locked and by the passed in thread
        returns activeThreads.remove(url); 
    }

    public synchonized void isURLStillLocked(String url)
    {
        returns activeThreads.contains(url); 
    }

}

有人对此有更好的解决方案吗？我的解决方案看起来有效吗？是否有任何开源组件已经可以很好地做到这一点，我可以利用？

谢谢

原文

I have a webapp that needs to sometimes download some bytes from a url and package it up and send back to the requester. The downloaded bytes are stored for a little while so they can be reused if the same url is needed to be downloaded. I am trying to figure out how best to prevent the threads from downloading the same url at the same time if the requests come in at the same time. I was thinking of creating a class like below that would prevent the same url from being downloaded at the same time. If a url is unable to be locked then it either waits until its not locked anymore to try and download it as long as it does not exist after the unlock.

public class URLDownloader
{
    HashMap<String,String> activeThreads = new HashMap<String,String>();

    public synchronized void lockURL(String url, String threadID) throws UnableToLockURLException
    {
        if(!activeThreads.containsKey(url))
            activeThreads.put(url, threadID)
        else
            throw UnableToLockURLException()
    }

    public synchonized void unlockURL(String url, String threadID)
    {
        //need to check to make sure its locked and by the passed in thread
        returns activeThreads.remove(url); 
    }

    public synchonized void isURLStillLocked(String url)
    {
        returns activeThreads.contains(url); 
    }

}

Does anyone have a better solution for this? Does my solution seem valid? Are there any open source components out there that already do this very well that I can leverage?

Thanks

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强者自强 2024-12-28 02:41:48

我建议保留一个 ConcurrentHashSet 来跟踪所有线程可见的唯一 URL。此构造可能不直接存在于 java 库中，但可以通过 ConcurrentHashMap 轻松构造，如下所示：Collections.newSetFromMap(new ConcurrentHashMap())

回复收藏 0 原文

神爱温柔 2024-12-28 02:41:48

听起来你不需要锁，因为如果有多个请求下载同一个 URL，那么重点就是只下载一次。

另外，我认为就封装而言，将检查存储的 URL/例程以将新 URL 存储在 URLDownloader 类中而不是在调用类中更有意义。您的线程可以简单地调用例如 fetchURL()，并让 URLDownloader 处理具体细节。

因此，您可以通过两种方式实现这一点。如果您没有源源不断的下载请求，更简单的方法是仅运行一个 URLDownloader 线程，并使其 fetchURL 方法同步< /code>，这样您一次只能下载一个 URL。否则，将待处理的下载请求保留在中央 LinkedHashSet 中，这会保留顺序并忽略重复。