Please Come IN! IanSchneider. About random access serialized objects.

Question

Hi,I'm a freshman.
I have a question about serialized objects.
If I useing writeObject() serialize objects into a file in order,I can gain them with readObject() in the same order. But if I want to random access these serialized objects rather than one by one,how to do?
Mr IanSchneider said "write all your objects into the same file using normal io techniques and you can still generate an index and acheive random access". It seems easy,but how to generate index and use it to acheive random access? Please write something in detail,Thank you in advance!
EXPECTING��

Answer 1

Have a look at this class: [ [u]WARNING: I just wrote this code, it hasn't been tested ]
import java.io.BufferedInputStream;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.RandomAccessFile;
import java.io.Serializable;
import java.util.ArrayList;
public class SerializedObjectWriter {
     private RandomAccessFile raf;
     private String filepath;
     private String mode;
     public SerializedObjectWriter(String filePath, String mode) throws FileNotFoundException {                         
          this.filepath = filePath;
          this.mode = mode;
          this.raf = new RandomAccessFile(filePath, mode);                         
     public void writeObject(Object o, long pos) throws IOException {
          raf.seek(pos);          
          final byte[] bytes = serialize((Serializable)o);     
          raf.writeInt(bytes.length);     
          raf.write(bytes);
     public void append(Object o) throws IOException {                    
          writeObject(o, raf.length());
     public Object readObject(long pos) throws IOException, ClassNotFoundException {
          raf.seek(pos);
          int len = raf.readInt();          
          final byte[] data = new byte[len];          
          raf.readFully(data);
          return deSerialize(data);
     public Object[] readAllObjects() throws IOException, ClassNotFoundException {          
          int pos = 0;          
          final ArrayList al = new ArrayList();
          while (true) {
               raf.seek(pos);
               final int len = raf.readInt();               
               final byte[] data = new byte[len];          
               raf.readFully(data);
               al.add(deSerialize(data));
               pos = (pos + len + 4);               
               if (pos >= raf.length()) break;               
          return al.toArray();
     public long length() throws IOException {
          return raf.length();
     public void reset() throws IOException {          
          raf.close();
          final boolean success = new File(filepath).delete();
          if (!success) throw new IOException("Failed to delete file");
          raf = new RandomAccessFile(filepath, mode);
     public void close() throws IOException {          
          raf.close();     
     private byte[] serialize(Serializable obj) throws IOException {
          final ByteArrayOutputStream baos = new ByteArrayOutputStream();
          final ObjectOutputStream oos = new ObjectOutputStream(baos);
          try {
               oos.writeObject(obj);
          } finally {
               oos.close();
          return baos.toByteArray();
     private Object deSerialize(byte[] data) throws IOException, ClassNotFoundException {
          final ByteArrayInputStream bais = new ByteArrayInputStream(data);     
          final BufferedInputStream bis = new BufferedInputStream(bais);
          final ObjectInputStream ois = new ObjectInputStream(bis);
          try {
               return (Serializable) ois.readObject();               
          } finally {
               ois.close();
     public static void main(String[] args) throws Exception {
          SerializedObjectWriter sor = new SerializedObjectWriter("C:\\test.ser", "rw");          
          sor.reset();
          sor.writeObject("Test 1", 0);                         
          sor.append("Test 2");               
          sor.append("Test 3");     
          Object[] objects = sor.readAllObjects();
          for (int i = 0; i < objects.length; i++) {
               System.out.println(objects);
          sor.close();