Java中的大量常量

Question

I need to include about 1 MByte of data in a Java application, for very fast and easy access in the rest of the source code. My main background is not Java, so my initial idea was to convert the data directly to Java source code, defining 1MByte of constant arrays, classes (instead of C++ struct) etc., something like this:

public final/immutable/const MyClass MyList[] = { 
  { 23012, 22, "Hamburger"} , 
  { 28375, 123, "Kieler"}
};

However, it seems that Java does not support such constructs. Is this correct? If yes, what is the best solution to this problem?

NOTE: The data consists of 2 tables with each about 50000 records of data, which is to be searched in various ways. This may require some indexes later, with significant more records, maybe 1 million records, saved this way. I expect the application to start up very fast, without iterating through these records.

Answer 1

I personally wouldn't put it in source form.

Instead, include the data in some appropriate raw format in your jar file (I'm assuming you'll be packaging the application or library up) and use Class.getResourceAsStream or ClassLoader.getResourceAsStream to load it.

You may very well want a class to encapsulate loading, caching and providing this data - but I don't see much benefit from converting it into source code.

Answer 2

Due to limitations of the java bytecode files, class-files can not be larger than 64k iirc. (They are simply not intended for this type of data.)

I would load the data upon starting the program, using something like the following lines of code:

import java.io.*;
import java.util.*;

public class Test {
    public static void main(String... args) throws IOException {
        List<DataRecord> records = new ArrayList<DataRecord>();
        BufferedReader br = new BufferedReader(new FileReader("data.txt"));
        String s;
        while ((s = br.readLine()) != null) {
            String[] arr = s.split(" ");
            int i = Integer.parseInt(arr[0]);
            int j = Integer.parseInt(arr[1]);
            records.add(new DataRecord(i, j, arr[0]));
        }
    }
}


class DataRecord {
    public final int i, j;
    public final String s;
    public DataRecord(int i, int j, String s) {
        this.i = i;
        this.j = j;
        this.s = s;
    }
}

( NB: The Scanner is quite slow, so don't be tempted to use it just because it has a simple interface. Stick with some form of BufferedReader and split, or StringTokenizer.)

Efficiency can of course be improved if you transform the data into a binary format. In that case, you can make use of the DataInputStream (but don't forget to go through some BufferedInputStream or BufferedReader )

Depending on how you wish to access the data, you might be better off storing the records in a hash-map ( HashMap<Integer, DataRecord> ) (having i or j as the key).

If you wish to load the data at the same time as the JVM loads the class file itself (roughly!) you could do the read / initialization, not within a method, but ecapsulated in static { ... } .

---

For a memory-mapped approach , have a look at the java.nio.channels -package in java. Especially the method

public abstract MappedByteBuffer map(FileChannel.MapMode mode, long position,long size) throws IOException

Complete code examples can be found here .

---

Dan Bornstein (the lead developer of DalvikVM) explains a solution to your problem in this talk (Look around 0:30:00). However I doubt the solution applies to as much data as a megabyte.

Answer 3

An idea is that you use enumerators, but I'm not sure if this suits to your implementation, and it also depends on how you are planning to use the data.

public enum Stuff {

 HAMBURGER (23012, 22),
 KIELER    (28375, 123);

 private int a;
 private int b;

 //private instantiation, does not need to be called explicitly.
 private Stuff(int a, int b) {
    this.a = a;
    this.b = b;
  }

 public int getAvalue() {
   return this.a;
 }

 public int getBvalue() {
   return this.b;
 }

}

These can then be accessed like:

Stuff someThing = Stuff.HAMBURGER;
int hamburgerA = Stuff.HAMBURGER.getA() // = 23012

Another idea is using a static initializer to set private fields of a class.

Answer 4

Putting the data into source could would actually not be the fastest solution, not by a long shot. Loading a Java class is quite complex and slow (at least on a platform that does bytecode verification, not sure about Android).

The fastest possible way to do this would be to define your own binary index format. You could then read that as a byte[] (possibly using memory mapping) or even a RandomAccessFile without interpreting it in any way until you start accessing it. The cost of this would be the complexity of the code that accesses it. With fixed-size records, a sorted list of records that's accessed via binary search would still be pretty simple, but anything else is going to get ugly.

Though before doing that, are you sure this isn't premature optimization? The easiest (and probably still quite fast) solution would be to jsut serialize a Map, List or array - have you tried this and determined that it is, in fact, too slow?

Answer 5

convert the data directly to Java source code, defining 1MByte of constant arrays, classes

Be aware that there are strict constraints on the size of classes and their structures [ref JVM Spec .

Answer 6

This is how you define it in Java, if I understood what you are after:

public final Object[][] myList = { 
          { 23012, 22, "Hamburger"} , 
          { 28375, 123, "Kieler"}
        };

Answer 7

It looks like you plan to write your own lightweight database.
If you can limit the length of the String to a realistic max size the following might work:

• write each entry into a binary file, the entries have the same size, so you waste some bytes with each entry(int a, int b,int stringsize, string, padding)
• To read an entry open the file as a random access file, multiply the index with the length of an entry to get the offset and seek the position.
• Put the bytes into a bytebuffer and read the values, the String has to be converted with the String(byte[] ,int start, int length,Charset) ctor.

If you can't limit the length of a block dump the strings in an additional file and only store the offsets in your table. This requires an additional file access and makes modifiying the data hard.
Some informationa about random file-access in java can be found here http://java.sun.com/docs/books/tutorial/essential/io/rafs.html .

For faster access you can cache some of your read entries in a Hashmap and always remove the oldest from the map when reading a new one.
Pseudo code (wont compile):

class MyDataStore
{
   FileChannel fc = null;
   Map<Integer,Entry> mychace = new HashMap<Integer, Entry>();
   int chaceSize = 50000;
   ArrayList<Integer> queue = new ArrayList();
   static final int entryLength = 100;//byte
   void open(File f)throws Exception{fc = f.newByteChannel()}
   void close()throws Exception{fc.close();fc = null;}
   Entry getEntryAt(int index)
   {
       if(mychace.contains(index))return mychace.get(index);

       long pos = index * entryLength; fc.seek(pos);ByteBuffer 
       b = new ByteBuffer(100);
       fc.read(b);
       Entry a = new Entry(b);
       queue.add(index);
       mychace.put(index,a);
       if(queue.size()>chacesize)mychace.remove(queue.remove(0));
       return a;
   }

}
class Entry{
   int a; int b; String s;
   public Entry(Bytebuffer bb)
   {
     a = bb.getInt(); 
     b = bb.getInt(); 
     int size = bb.getInt();
     byte[] bin = new byte[size];
     bb.get(bin);
     s = new String(bin);
   }
}

Missing from the pseudocode:

• writing, since you need it for constant data
• total number of entries/sizeof file, only needs an additional integer at the beginning of the file and an additional 4 byte offset for each access operation.

Answer 8

我建议使用资产来存储这些数据。

Answer 9

You could also declare a static class (or a set of static classes) exposing the desidered values as methods . After all, you want your code to be able to find the value for a given name, and don't want the value to change.

So: location=MyLibOfConstants.returnHamburgerLocation().zipcode

And you can store this stuff in a hashtable with lazyinitialization, if you thing that calculating it on the fly would be a waste of time.

Answer 10

Isn't a cache what you need? As classes it is loaded in the memory, not really limited to a defined size, should be as fast as using constants... Actually it can even search data with some kind of indexes (exemple with the object hashcode...) You can for exemple create all your data arrays (ex { 23012, 22, "Hamburger"}) and then create 3 hashmap: map1.put(23012,hamburgerItem); map2.put(22,hamburgerItem); map3.put("Hamburger",hamburgerItem); This way you can search very fast in one of the map according to the parameter you have... (but this works only if your keys are unique in the map... this is just an exemple that could inspire you)

At work we have a very big webapp (80 weblogic instances) and it's almost what we do: caching everywhere. From a countrylist in database, create a cache...

There are many different kind of caches, you should check the link and choose what you need... http://en.wikipedia.org/wiki/Cache_algorithms

Answer 11

Java serialization sounds like something that needs to be parsed... not good. Isn't there some kind of standard format for storing data in a stream, that can be read/looked up using a standard API without parsing it?

If you were to create the data in code, then it would all be loaded on first use. This is unlikely to be much more efficient than loading from a separate file - as well as parsing the data in the class file, the JVM has to verify and compile the bytecodes to create each object a million times, rather than just the once if you load it from a loop.

If you want random access and can't use a memory mapped file, then there is a RandomAccessFile which might work. You need either to load a index on start, or you need to make the entries a fixed length.

You might want to check whether the HDF5 libraries run on your platform; it may be overkill for such a simple and small dataset though.