Question

我有一个RecyclerView，其中包含文件列表。用户可以点击许多RecyclerView行，然后上传所选文件。一旦文件被上传（在后台线程上），我想从RecyclerView中删除该行。

但是，我收到错误消息：

java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.next(ArrayList.java:860)
    at UploadActivity$11$1.onFinish(UploadActivity.java:471)
    at FileUploader.upload(FileUploader.java:115)
    at UploadActivity$11.run(UploadActivity.java:458)

我了解我有多个线程同时访问和修改RecyclerView适配器，但是我不确定如何解决此问题（我的代码和尝试都发布在下面）。

首先，我循环浏览选定的RecyclerView项，获取每个选定的文件，然后调用upload（）

// Loop through all selected RecyclerView items 
for (int i = 0; i < selectedIndicies.size(); i++) {

    // Get the i-th selected item
    Upload_Item_Model selectedItem = adapter.getFilteredData().get(selectedIndicies.get(i));

    // Get the file associated with the i-th selected item
    SaveFile file = getFileWithFilename(token, selectedItem.getTitle(), UploadActivity.this);

    // Upload the file
    uploadFile(file);
}

然后我开始一个新线程，开始上载，并定义onFinish（）回调

public void uploadFile(SaveFile saveFile) {

    ...

    new Thread() {

        @Override
        public void run() {

            // 
            // Uploads the given file, when the upload is complete
            // the onFinish() method is called and the file is passed
            // back so I can update the RecyclerView
            //

            FileUploader.upload(saveFile, new FileUploader.FileUploadListener() {

                @Override
                public void onFinish(SaveFile file) {

                    // Loop through all items in RecyclerView
                    for (Upload_Item_Model item : adapter.getFilteredData()) { // this is line 471 where the crash happens

                        //
                        // If the RecyclerView item has the same name
                        // as the returned file, then it is 
                        // the file I just uploaded
                        //

                        if (item.getTitle().equals(file.getFilename())) {
                            runOnUiThread(() -> {

                                // Removes the item from the adapter
                                adapter.removeItem(item);
                                adapter.notifyDataSetChanged();
                            });

                        }
                    }
                }
            });
        }
    }.start();
}

在适配器中，我具有以下功能，用于访问和修改适配器ArrayList。我试图使这些线程安全无事。

public ArrayList<Upload_Item_Model> getFilteredData() {
    synchronized (this.filteredData) {
        return this.filteredData;
    }
}

public void removeItem(Upload_Item_Model item) {
    synchronized (this.filteredData) {
        this.filteredData.remove(item);
    }
}

任何帮助或建议都值得赞赏！

编辑+解决方案

我使用Rajat Mehra的解决方案使所有工作正常进行，该解决方案使用一个线程上载所有文件，而不是使用多个线程上载一个文件。为了使它正常工作，我确实需要进行一些小的调整，但是现在一切都顺利进行。

public void uploadFile() {

    new Thread() {

        @Override
        public void run() {

            for (int i = 0; i < selectedIndicies.size(); i++) {

                Upload_Item_Model selectedItem = adapter.getFilteredData().get(selectedIndicies.get(i));

                SaveFile file = getFileWithFilename(token, selectedItem.getTitle(), UploadActivity.this);
                FileUploader.upload(file, new FileUploader.FileUploadListener() {

                    @Override
                    public void onFinish() {

                        runOnUiThread(() -> {

                            // I can now simply use the selectedItem here!
                            adapter.removeItem(selectedItem);
                            adapter.notifyDataSetChanged();
                        });
                    }
                });
            }
        }
    }.start();
}

Answer 1

创建一个线程并在其中一个接一个地上传所有文件，而不是创建多个线程。

public void uploadFile() {


    new Thread() {

        @Override
        public void run() {

            //
            // Uploads the given file, when the upload is complete
            // the onFinish() method is called and the file is passed
            // back so I can update the RecyclerView
            //
            for (int i = 0; i < selectedIndicies.size(); i++) {
                // Get the i-th selected item
                Upload_Item_Model selectedItem = adapter.getFilteredData().get(selectedIndicies.get(i));

                // Get the file associated with the i-th selected item
                SaveFile file = getFileWithFilename(token, selectedItem.getTitle(), UploadActivity.this);
                FileUploader.upload(file, new FileUploader.FileUploadListener() {

                    @Override
                    public void onFinish(SaveFile file) {

                        // Loop through all items in RecyclerView
                        for (Upload_Item_Model item : adapter.getFilteredData()) { // this is line 471 where the crash happens

                            //
                            // If the RecyclerView item has the same name
                            // as the returned file, then it is
                            // the file I just uploaded
                            //

                            if (item.getTitle().equals(file.getFilename())) {
                                runOnUiThread(() -> {

                                    // Removes the item from the adapter
                                    adapter.removeItem(item);
                                    adapter.notifyDataSetChanged();
                                });

                            }
                        }
                    }
                });

            }
        }
    }.start();
}

Answer 2

调用adapter.notifyDataSetChanged();时会引发异常，因为在notifyDataSetChanged的后台处理了一些东西（没有同步），而同时在调用this.filteredData.remove(item);时却没有不论是否包含同步关键字。

可能首先您需要致电所有removeItem，然后致电adapter.notifyDataSetChanged(); 像这样：

for (Upload_Item_Model item: adapter.getFilteredData()) {
 if (item.getTitle().equals(file.getFilename())) {
  runOnUiThread(() -> {
   adapter.removeItem(item);
  });

 }
}
adapter.notifyDataSetChanged();

要考虑：notifyDataSetChanged()不是更新回收者视图项目的有效方法。有DiffUtil.Callback，可检查两个数据集之间的差异并有效地将更改分发到回收者视图。

从多个线程修改ArrayAdapter时如何防止ConcurrentModificationException？

2 个答案: