fileupload的使用

好久没有搞过上传的东西了，最近要对一个系统进行“改装”一下，原来用到的好多东西都要更换一下，并增加一些新功能，用到了fileupload，于是就重新学习了一下。一下是从apache官网粘帖过来的，写的不错。绝对够用了。

 

Parsing the request

Before you can work with the uploaded items, of course, you need to parse the request itself. Ensuring that the request is actually a file upload request is straightforward, but FileUpload makes it simplicity itself, by providing a static method to do just that.

// Check that we have a file upload request
boolean isMultipart = ServletFileUpload.isMultipartContent(request);

Now we are ready to parse the request into its constituent items.

The simplest case

The simplest usage scenario is the following:

• Uploaded items should be retained in memory as long as they are reasonably small.
• Larger items should be written to a temporary file on disk.
• Very large upload requests should not be permitted.
• The built-in defaults for the maximum size of an item to be retained in memory, the maximum permitted size of an upload request, and the location of temporary files are acceptable.

 

Handling a request in this scenario couldn't be much simpler:

// Create a factory for disk-based file items
FileItemFactory factory = new DiskFileItemFactory();

// Create a new file upload handler
ServletFileUpload upload = new ServletFileUpload(factory);

// Parse the request
List /* FileItem */ items = upload.parseRequest(request);

That's all that's needed. Really!

The result of the parse is a List of file items, each of which implements the FileItem interface. Processing these items is discussed below.

Exercising more control

If your usage scenario is close to the simplest case, described above, but you need a little more control, you can easily customize the behavior of the upload handler or the file item factory or both. The following example shows several configuration options:

// Create a factory for disk-based file items
DiskFileItemFactory factory = new DiskFileItemFactory();

// Set factory constraints
factory.setSizeThreshold(yourMaxMemorySize);
factory.setRepository(yourTempDirectory);

// Create a new file upload handler
ServletFileUpload upload = new ServletFileUpload(factory);

// Set overall request size constraint
upload.setSizeMax(yourMaxRequestSize);

// Parse the request
List /* FileItem */ items = upload.parseRequest(request);

Of course, each of the configuration methods is independent of the others, but if you want to configure the factory all at once, you can do that with an alternative constructor, like this:

// Create a factory for disk-based file items
DiskFileItemFactory factory = new DiskFileItemFactory(
        yourMaxMemorySize, yourTempDirectory);

Should you need further control over the parsing of the request, such as storing the items elsewhere - for example, in a database - you will need to look into customizing FileUpload.

Processing the uploaded items

Once the parse has completed, you will have a List of file items that you need to process. In most cases, you will want to handle file uploads differently from regular form fields, so you might process the list like this:

// Process the uploaded items
Iterator iter = items.iterator();
while (iter.hasNext()) {
    FileItem item = (FileItem) iter.next();

    if (item.isFormField()) {
        processFormField(item);
    } else {
        processUploadedFile(item);
    }
}

For a regular form field, you will most likely be interested only in the name of the item, and its String value. As you might expect, accessing these is very simple.

// Process a regular form field
if (item.isFormField()) {
    String name = item.getFieldName();
    String value = item.getString();
    ...
}

For a file upload, there are several different things you might want to know before you process the content. Here is an example of some of the methods you might be interested in.

// Process a file upload
if (!item.isFormField()) {
    String fieldName = item.getFieldName();
    String fileName = item.getName();
    String contentType = item.getContentType();
    boolean isInMemory = item.isInMemory();
    long sizeInBytes = item.getSize();
    ...
}

With uploaded files, you generally will not want to access them via memory, unless they are small, or unless you have no other alternative. Rather, you will want to process the content as a stream, or write the entire file to its ultimate location. FileUpload provides simple means of accomplishing both of these.

// Process a file upload
if (writeToFile) {
    File uploadedFile = new File(...);
    item.write(uploadedFile);
} else {
    InputStream uploadedStream = item.getInputStream();
    ...
    uploadedStream.close();
}

Note that, in the default implementation of FileUpload, write() will attempt to rename the file to the specified destination, if the data is already in a temporary file. Actually copying the data is only done if the the rename fails, for some reason, or if the data was in memory.

If you do need to access the uploaded data in memory, you need simply call the get() method to obtain the data as an array of bytes.

// Process a file upload in memory
byte[] data = item.get();
...

Resource cleanup

This section applies only, if you are using the DiskFileItem . In other words, it applies, if your uploaded files are written to temporary files before processing them.

Such temporary files are deleted automatically, if they are no longer used (more precisely, if the corresponding instance of java.io.File is garbage collected. This is done silently by theorg.apache.commons.io.FileCleaner class, which starts a reaper thread.

This reaper thread should be stopped, if it is no longer needed. In a servlet environment, this is done by using a special servlet context listener, called FileCleanerCleanup . To do so, add a section like the following to your web.xml:

<web-app>
  ...
  <listener>
    <listener-class>
      org.apache.commons.fileupload.servlet.FileCleanerCleanup
    </listener-class>
  </listener>
  ...
</web-app>

Creating a DiskFileItemFactory

The FileCleanerCleanup provides an instance of org.apache.commons.io.FileCleaningTracker. This instance must be used when creating aorg.apache.commons.fileupload.disk.DiskFileItemFactory. This should be done by calling a method like the following:

    public static DiskFileItemFactory newDiskFileItemFactory(ServletContext context,
                                                             File repository) {
        FileCleaningTracker fileCleaningTracker
            = FileCleanerCleanup.getFileCleaningTracker(context);
        return new DiskFileItemFactory(fileCleaningTracker,
                                       DiskFileItemFactory.DEFAULT_SIZE_THRESHOLD,
                                       repository);
    }

Disabling cleanup of temporary files

To disable tracking of temporary files, you may set the FileCleaningTracker to null. Consequently, created files will no longer be tracked. In particular, they will no longer be deleted automatically.

Interaction with virus scanners

Virus scanners running on the same system as the web container can cause some unexpected behaviours for applications using FileUpload. This section describes some of the behaviours that you might encounter, and provides some ideas for how to handle them.

The default implementation of FileUpload will cause uploaded items above a certain size threshold to be written to disk. As soon as such a file is closed, any virus scanner on the system will wake up and inspect it, and potentially quarantine the file - that is, move it to a special location where it will not cause problems. This, of course, will be a surprise to the application developer, since the uploaded file item will no longer be available for processing. On the other hand, uploaded items below that same threshold will be held in memory, and therefore will not be seen by virus scanners. This allows for the possibility of a virus being retained in some form (although if it is ever written to disk, the virus scanner would locate and inspect it).

One commonly used solution is to set aside one directory on the system into which all uploaded files will be placed, and to configure the virus scanner to ignore that directory. This ensures that files will not be ripped out from under the application, but then leaves responsibility for virus scanning up to the application developer. Scanning the uploaded files for viruses can then be performed by an external process, which might move clean or cleaned files to an "approved" location, or by integrating a virus scanner within the application itself. The details of configuring an external process or integrating virus scanning into an application are outside the scope of this document.

Watching progress

If you expect really large file uploads, then it would be nice to report to your users, how much is already received. Even HTML pages allow to implement a progress bar by returning a multipart/replace response, or something like that.

Watching the upload progress may be done by supplying a progress listener:

//Create a progress listener
ProgressListener progressListener = new ProgressListener(){
   public void update(long pBytesRead, long pContentLength, int pItems) {
       System.out.println("We are currently reading item " + pItems);
       if (pContentLength == -1) {
           System.out.println("So far, " + pBytesRead + " bytes have been read.");
       } else {
           System.out.println("So far, " + pBytesRead + " of " + pContentLength
                              + " bytes have been read.");
       }
   }
};
upload.setProgressListener(progressListener);

Do yourself a favour and implement your first progress listener just like the above, because it shows you a pitfall: The progress listener is called quite frequently. Depending on the servlet engine and other environment factory, it may be called for any network packet! In other words, your progress listener may become a performance problem! A typical solution might be, to reduce the progress listeners activity. For example, you might emit a message only, if the number of megabytes has changed:

//Create a progress listener
ProgressListener progressListener = new ProgressListener(){
   private long megaBytes = -1;
   public void update(long pBytesRead, long pContentLength, int pItems) {
       long mBytes = pBytesRead / 1000000;
       if (megaBytes == mBytes) {
           return;
       }
       megaBytes = mBytes;
       System.out.println("We are currently reading item " + pItems);
       if (pContentLength == -1) {
           System.out.println("So far, " + pBytesRead + " bytes have been read.");
       } else {
           System.out.println("So far, " + pBytesRead + " of " + pContentLength
                              + " bytes have been read.");
       }
   }
};

在此记录一下，最近比较郁闷，不知道为什么有时候就登陆不了javaeye，还有javaeye的编辑器，我怎么都不会用，还有那个附件上传的，word都不让传。。闹心。希望改进一下，不然我就要搬家了。O(∩_∩)O哈！