请使用Android jetPack 框架workmanager实现一个后台拷贝多个文件到U盘得功能，要求有后台百分比进度条显示拷贝进度，语言请使用Java

首先，需要在项目中引入WorkManager库：

implementation 'androidx.work:work-runtime:2.4.0'

然后，创建一个Worker类实现文件拷贝的任务。在doWork()方法中执行拷贝任务，并使用setProgressAsync()方法设置进度：

public class FileCopyWorker extends Worker {
    private static final String TAG = "FileCopyWorker";

    public FileCopyWorker(@NonNull Context context, @NonNull WorkerParameters workerParams) {
        super(context, workerParams);
    }

    @NonNull
    @Override
    public Result doWork() {
        String[] filesToCopy = getInputData().getStringArray("files_to_copy");
        String usbDrivePath = getInputData().getString("usb_drive_path");
        if (filesToCopy == null || usbDrivePath == null) {
            Log.e(TAG, "Invalid input data");
            return Result.failure();
        }

        int totalFiles = filesToCopy.length;
        int filesCopied = 0;
        for (String fileToCopy : filesToCopy) {
            // Copy fileToCopy to usbDrivePath
            // ...
            filesCopied++;
            setProgressAsync(new Data.Builder().putInt("progress", (int) (filesCopied * 100.0f / totalFiles)).build());
        }

        return Result.success();
    }
}

接下来，在Activity中创建WorkRequest并提交任务：

// Create WorkRequest
Data inputData = new Data.Builder()
        .putStringArray("files_to_copy", new String[] {"/path/to/file1", "/path/to/file2"})
        .putString("usb_drive_path", "/mnt/usb")
        .build();
OneTimeWorkRequest fileCopyRequest = new OneTimeWorkRequest.Builder(FileCopyWorker.class)
        .setInputData(inputData)
        .build();

// Submit WorkRequest
WorkManager.getInstance(context).enqueue(fileCopyRequest);

最后，在Activity中监听任务进度并更新UI：

WorkManager.getInstance(context).getWorkInfoByIdLiveData(fileCopyRequest.getId())
        .observe(this, workInfo -> {
            if (workInfo != null && workInfo.getState() == WorkInfo.State.RUNNING) {
                int progress = workInfo.getProgress().getInt("progress", 0);
                progressBar.setProgress(progress);
            }
        });

以上是一个简单的示例，可以根据实际需求进行修改和扩展。