Reading picture from byte array

Question

I'm writing a Java applet that need to read some XML and image files inside a ZIP file. The Zip file will be download through HTTP and the applet is unsigned, so I need to use java.util.zip.ZipInputStream to manipulated the data. There's a problem when I'm trying to read a PNG file inside the Zip file.

The steps I handle the Zip file:

1. Download the Zip through Http

   URL resourceURL = new URL(source);
   HttpURLConnection httpConnection= (HttpURLConnection) resourceURL.openConnection();
   httpConnection.connect();
   InputStream resourceFileIn = httpConnection.getInputStream();
   

2. Use ZipInputStream to hold the data downloaded

   ZipInputStream resourceZipIn = new ZipInputStream(resourceFileIn);
   

3. Iterate through every ZipEntry and extract the corresponding byte array

   ArrayList<ExtractedEntry> extractedList = new ArrayList<ExtractedEntry>();
   ZipEntry currentEntry;
   while ((currentEntry = resourceZipIn.getNextEntry()) != null) {
       byte[] byteHolder = new byte[(int) currentEntry.getSize()];
       resourceZipIn.read(byteHolder, 0, byteHolder.length);
       extractedList.add(new ExtractedEntry(currentEntry.getName(), byteHolder));
   }
   

   remarks: every ZipEntry extracted is hold by the following class

   public class ExtractedEntry {
       private String name;
       private byte[] byteArray;
   
       public ExtractedEntry(String name, byte[] byteArray) {
           super();
           this.name = name;
           this.byteArray = byteArray;
       }
   
       public String getName() {
           return name;
       }
   
       public byte[] getByteArray() {
           return byteArray;
       }
   }
   

4. Find the file I want to read from in the extracted list

   ExtractedEntry bgEntry = null;
   for (int j = 0; j < extractedList.size() && bgEntry == null; j++) {
       if (extractedList.get(j).getName().equals("background.png")) {
           bgEntry = extractedList.get(j);
       }
   }
   

5. Perform specific action according to need

   InputStream mapBgIn = new ByteArrayInputStream(bgEntry.getByteArray());
   BufferedImage bgEntry = ImageIO.read(bgIn);
   

---

I list out only the action of reading the PNG file as that is where I encounter the problem. When I try to read the image in the way mentioned above, I always receive the following error at the last line of step 5.

javax.imageio.IIOException: Error reading PNG image data at com.sun.imageio.plugins.png.PNGImageReader.readImage(Unknown Source)
    at com.sun.imageio.plugins.png.PNGImageReader.read(Unknown Source)
    at javax.imageio.ImageIO.read(Unknown Source)
    at javax.imageio.ImageIO.read(Unknown Source)
    at com.quadd.soft.game.wtd.lib.resource.ResourceManager.loadHttpZipRes(ResourceManager.java:685)
Caused by: javax.imageio.IIOException: Unknown row filter type (= 7)!
    at com.sun.imageio.plugins.png.PNGImageReader.decodePass(Unknown Source)
    at com.sun.imageio.plugins.png.PNGImageReader.decodeImage(Unknown Source)
    ... 29 more

---

However, if I read the image in the following way starting from step 3, there's no problem.

ZipInputStream resourceZipIn = new ZipInputStream(resourceFileIn);
ZipEntry testEntry;
while ((testEntry = resourceZipIn.getNextEntry()) != null) {
    if (testEntry.getName().equals("background.png")) {
        BufferedImage bgEntry = ImageIO.read(resourceZipIn);
    }
}

---

Therefore, I guess that there's some problem with my code at extracting the bytes from java.util.zip.ZipInputStream and put it back for reading the image. But I'm quite new to manipulating stream so I just couldn't figure out what exactly is the problem. I would like to ask if anyone could point out what mistake I've made in the code which cause the error.

Answer 1

The read method does not guarantee that it fills the byte array; it reads only a small block and returns the number of bytes that it read. You need a loop, or use a helper class that does fill the array.

For example

DataInputStream in = new DataInputStream(resourceZipIn);
in.readFully(byteHolder);
in.close();