java - Writing a BufferedImage cache and save it to disk -

i'm developing java application in load long lists containing images (downloaded web), added quick hashmap<string,bufferedimage> cache, avoid redownloading same image multiple times.

this works fine , application way faster, nice let cache persist through various sessions, changed cache serialized.

bufferedimage not serializable, had wrote custom methods.

my file structure should like:

• (int) number of elements
• [(url) image's key
• (object) image written using imageio] n times

while file saving seems fine (at least have no exceptions), when try load url throws java.io.optionaldataexception length = 4 , don't understand why. first iteration goes fine, have exception try load second url, suspect there's wrong in way load first image.

here's full code :

import java.awt.image.bufferedimage; import java.io.file; import java.io.fileinputstream; import java.io.fileoutputstream; import java.io.ioexception; import java.io.objectinputstream; import java.io.objectoutputstream; import java.net.url; import java.util.hashmap; import java.util.map.entry; import java.util.logging.level; import java.util.logging.logger; import javax.imageio.imageio;   public class picturescache {      private static hashmap<string, bufferedimage> picturescache;      private static final string cachedisklocation = "pictures_cache.map";      private static void writecache(objectoutputstream oos, hashmap<string, bufferedimage> data) throws ioexception {         // number of saved elements         oos.writeint(data.size());         // let's write (url, image) each entry in cache         (entry<string, bufferedimage> entry : data.entryset()) {                oos.writeobject(new url(entry.getkey()));             imageio.write(entry.getvalue(), "png", oos);         }     }      private static hashmap<string, bufferedimage> readcache(objectinputstream ois) throws ioexception, classnotfoundexception {         // number of saved elements         int size = ois.readint();         // cache         hashmap<string, bufferedimage> result = new hashmap<>(size);         // let's read (url, image) , add them cache         (int = 0; < size; i++) {             string url = ((url) ois.readobject()).tostring(); // exception here             bufferedimage image = imageio.read(ois);                         result.put(url, image);         }         return result;     }      public static void loadcache() {         picturescache = new hashmap<>();         file file = new file(cachedisklocation);         if (file.isfile()) {             fileinputstream fis = null;             objectinputstream ois = null;             try {                 fis = new fileinputstream(file);                 ois = new objectinputstream(fis);                 picturescache = readcache(ois);             } catch (ioexception | classnotfoundexception ex) {                 logger.getlogger(picturescache.class.getname()).log(level.severe, null, ex);             } {                 try {                     ois.close();                     fis.close();                 } catch (ioexception ex) {                     logger.getlogger(picturescache.class.getname()).log(level.severe, null, ex);                 }             }         }         system.out.println("cache loaded " + picturescache.size() + " elements");     }      public static void savecache() {         file file = new file(cachedisklocation);         fileoutputstream fos = null;         objectoutputstream oos = null;         try {             if (file.isfile()) {                 file.delete();             }             file.createnewfile();             fos = new fileoutputstream(file);             oos = new objectoutputstream(fos);             writecache(oos, picturescache);         } catch (ioexception ex) {             logger.getlogger(picturescache.class.getname()).log(level.severe, null, ex);         } {             try {                 system.out.println("cache saved " + picturescache.size() + " elements");                 oos.close();                 fos.close();             } catch (ioexception ex) {                 logger.getlogger(picturescache.class.getname()).log(level.severe, null, ex);             }         }     }      public static boolean contains(string url) {         return picturescache.containskey(url);     }      public static bufferedimage get(string url) {         return picturescache.get(url);     }      public static void put(string url, bufferedimage image) {         picturescache.put(url, image);     }  } 

the error occurs because imageio.read(...) doesn't read data written using imageio.write(...). can write image objectoutputstread byte[]. example:

private static void writecache(objectoutputstream oos,         hashmap<string, bufferedimage> data) throws ioexception {     // number of saved elements     oos.writeint(data.size());     // let's write (url, image) each entry in cache     (entry<string, bufferedimage> entry : data.entryset()) {         oos.writeobject(new url(entry.getkey()));         bytearrayoutputstream baos = new bytearrayoutputstream();         imageio.write(entry.getvalue(), "jpg", baos);         byte[] bytes = baos.tobytearray();         oos.writeobject(bytes);     } }  private static hashmap<string, bufferedimage> readcache(         objectinputstream ois) throws ioexception, classnotfoundexception {     // number of saved elements     int size = ois.readint();     // cache     hashmap<string, bufferedimage> result = new hashmap<>(size);     // let's read (url, image) , add them cache     (int = 0; < size; i++) {         string url = ((url) ois.readobject()).tostring(); // exception here         bytearrayinputstream bais = new bytearrayinputstream(                 (byte[]) ois.readobject());         bufferedimage image = imageio.read(bais);         result.put(url, image);     }     return result; }