FAQ for JavaGDBM

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This is not an official publication of Pharos. There is no warranty of any kind on this software or document.

Is this a JDBC driver?

(Alain Magloire)

No. JDBC seems mainly intended for interfaces to relational databases, and GDBM is not a relational database.

However, JDBC does permit use of a subset to interface to non-relational databases. Implementing this might be interesting: let me know if it's useful to you.

Does JavaGDBM understand SQL?

(Alain Magloire)

No, JavaGDBM is intended to store on-disk hash tables. They solve a different class of problem to that appropriate for SQL and relational databases.

What type of database should I use?

Broadly, options for data storage in Java include

Make sure you choose the right tools: there is no single `best' answer.

Are you maintaining GDBM?

No. I understand that GDBM is still maintained by the FSF.

Does it work on NT?

It should, but I haven't tried it. You'll need a C compiler to build the native library. Let me know if you try.

Why does the test harness report exceptions

The test harness may report messages such as 

caught: au.com.pharos.gdbm.GdbmException: File open error: No such file or
directory? (gdbmjava.c:101)

These are expected exceptions: the test harness is making sure that exceptions are thrown when they should be.

I was trying to compile the Java GDBM distribution and it says gdbm.h not found

(Sandeep Kumar)

  1. Make sure you have the GDBM library installed on your system. If the library is not installed, you should be able to find it at www.gnu.org
  2. Make sure your compiler is configured to find the libraries and include files.

GdbmFile.h is missing

(Paul Matthews)

GdbmFile.h is build by javah when the library is built.

What is the best source to learn Java?

(Celil Germeyan)

Personally, I recommend the excellent Java series of books from O'Reilly and Associates and the Javasoft/Addison Wesley books.

I get a `not ok 40' message running the test

(Steve Waldman)

If you find a bug, either have a look at the code yourself, or send e-mail to [email protected]. There's no warranty on JavaGDBM, but I will try to respond to your mail and problem.

I get linker errors on Solaris

(Alex Soto) 

Hi,

    I'm hoping to use your JavaGDBM api for my application, but I'm
having some problems compiling the libgdbmjava.so.  Here is a partial
listing of the errors:

gcc -o libgdbmjava.so -shared gdbmjava.o -lgdbm
Text relocation remains                       referenced
    against symbol                  offset      in file
free                                0x258
/usr/local/lib/libgdbm.a(gdbmreorg.o)
free                                0xd0
/usr/local/lib/libgdbm.a(gdbmclose.o)
free                                0xb8
/usr/local/lib/libgdbm.a(gdbmclose.o)
free                                0x218
[snipped]

I have used the libgdbm.a lib before, and don't believe there is a
problem with it.  I'm compiling to Solaris 2.6, using JDK 1.1.4.

The solution turned out to be that the gdbm native library must be compiled with the -fpic flag to cause the compiler to generate position-independant code.

What happens if a reader tries to open the database while a writer is doing something?

(Rich Neill)

Access will be denied until the writer releases its lock.

JavaGDBM uses a shared-read/exclusive-write protocol at the OS level. However, multiple Java threads can access a file concurrently.

What platforms does JavaGDBM run on?

It should run on any platform that supports JDK1.1 or higher, JNI (Java Native Interface), and GDBM. JavaGDBM is known to run on

If you have any information on other platforms, please let me know.

You don't anticipate going Pure Java soon, do you?

(Brian Thorstad)

It's on the `long-term' TODO list. I'd like to do it, but don't have any date in mind.

Acknowledgements

Everybody who's sent in questions or comments.

Redhat and Javasoft, and the authors of GDBM, Linux, gcc, autoconf, and other software used in building JavaGDBM.