sg-cdb Java库，用于使用cdb数据库

Question

I have downloaded sg-cdb Java library from http://www.strangegizmo.com/products/sg-cdb/ and included it in my project. I'm trying to read CDB database with the following code:

import com.strangegizmo.cdb.Cdb;
import com.strangegizmo.cdb.CdbElement;
import java.util.Enumeration;

class Start
{ 
    public static void main(String args[])
    {
        Cdb cdbFile = null;
        try{
            cdbFile = new Cdb("basefile.cdb");
        }catch (Exception e) { e.printStackTrace(); }

        try
        {
            Enumeration em = cdbFile.elements("basefile.cdb");
            while(em.hasMoreElements())
            {
                CdbElement cdbElt = (CdbElement) em.nextElement();
            }
        }catch (Exception e) {e.printStackTrace();}
    }
}

What am I doing wrong that I'm getting the following exception message:

java.lang.IllegalArgumentException: invalid cdb format
    at com.strangegizmo.cdb.Cdb$1.nextElement(Cdb.java:358)

Can you provide any example of working with this library?

Thank you.

Answer 1

I am the author of sg-cdb and can help you get going with the library.

The primary issue with your sample is that you have to "make" a CDB file before you can enumerate over the file's elements. The portable way to do that is to generate a file in the cdbmake/cdbdump format and then use the static CdbMake.make(...) method to turn that file into a constant database. You can then load that constant database with the Cdb class and query for keys, enumerate over values, etc.

sg-cdb supports another mechanism for creating constant databases, which is to construct an instance of the CdbMake class directly, call start(...) to begin writing to the database, call add(...) to add keys to the database, and then call finish() when you are done writing keys.

Here is an update to your example that uses CdbMake.start/.add/.finish to generate a file and then enumerate over the file:

import com.strangegizmo.cdb.Cdb;
import com.strangegizmo.cdb.CdbElement;
import com.strangegizmo.cdb.CdbMake;
import java.util.Enumeration;

class Start
{ 
    public static void main(String args[])
    {
        try{
            CdbMake maker = new CdbMake();
            maker.start("basefile.cdb");
            maker.add("one".getBytes(), "Hello".getBytes());
            maker.add("two".getBytes(), "Goodbye".getBytes());
            maker.finish();
        }catch (Exception e) {e.printStackTrace();}

        try
        {
            Enumeration em = Cdb.elements("basefile.cdb");
            while(em.hasMoreElements())
            {
                CdbElement cdbElt = (CdbElement) em.nextElement();
            }
        }catch (Exception e) {e.printStackTrace();}
    }
}

Note that elements(...) is a static method on the Cdb class, so you do not need a Cdb instance in order to dump everything in the database.

In general my recommendation is that you ingest cdbdump -formatted files though. You would get the same results if you run cdbmake (or the cdb.make utility that is included with sg-cdb) on the following text file:

+3,5:one->Hello
+3,7:two->Goodbye

One final note: creating a new instance of the Cdb class always succeeds (even if the file does not exist), because it is perfectly valid to have a file with no keys. A Cdb instance initialized with a non-existent file (as you have done in your example) will correctly return null in response to every find() call. Cdb.enumerate(...) is not quite as forgiving though and expects to be given the path to a valid CDB file, which is why you are getting that IllegalArgumentException.

Let me know if you need any additional information!