Java and Postgres95

Java's native methods are functions written in C (or another compiled language) and dynamically loaded by the Java interpreter at run time. They provide the means to access libraries that have not been ported to Java, and also allow fast compiled code to be inserted at critical points in your system.

In this article, we will walk through the complete process of writing native code. We will create a Java interface to Postgres95 by writing wrapper classes around the libpq library. Postgres95 is a free database system (licensed under the GPL) that runs on most varieties of Unix, including Linux

While written (and tested) solely on Linux, the principles of this article should apply to any version of Unix and (with the exception of how to build the shared library) the code should be easily ported. To get the most out of this article, you should have some Java experience, or be very familiar with C++ and OO principles.

Recently, Java has received a great deal of attention (and quite a bit of hype) as a fantastic WWW tool. “Java Powered” pages with animations and interactive interfaces have popped up all over the Web, and everyone, including Microsoft (gasp!), is clamoring to add Java capabilities to their browsers. What many people don't realize is that Java is much more than that: it is a complete programming language suitable for use in standalone and, in particular, client-server applications.

Java offers several features that make it ideal for an application language. First among these is obviously portability. With Java there is no need to write Windows95, Mac, and several Unix versions of your application. Since the code is run by the Java Virtual Machine (VM), all that is necessary is that the VM (and any native libraries you want to use) be ported to that platform.

Another compelling reason to write in Java is the depth of its libraries (“packages” in Java-speak): networking, I/O, containers, and a complete windowing system are all integrated. Many of these capabilities are “crippled” when running a Java applet, but applications are free to make complete use of all of them. Java is a multi-threaded environment, allowing safe use of threads on platforms that don't currently support them natively. Java has a garbage collection system that eliminates the need for explicit freeing of memory. Exception handling is built in (and its use is actually required by many of the libraries, including the one we will write), and its true OO nature eases inheritance and re-use.

Sidebar: Class Repositories: the Motivation behind Jgres

Even with all these things going for it, using Java for an application still has one major drawback: many systems don't yet have a Java interface, and writing them from scratch is often difficult, or even impossible.

This is the problem I faced when I wanted to access a Postgres95 database from Java. There was an excellent (and simple) C library (libpq) that shipped with Postgres95, but no support whatsoever for Java. Since the source (in this case) was available, I considered recreating libpq under Java, but this proved to be a substantial chore, and required intimate knowledge of Postgres internals. (In fact, as of this writing, John Kelly of the Blackdown Organization is writing just such a beast. It's called the Java-Postgres95 project, and you can find an alpha version at ftp://java.blackdown.org/pub/Java.

Then I decided to simply write wrapper classes for libpq. There are several drawbacks to this approach: First, it cannot be used in an applet. Browsers explicitly disallow any access to native code (except those provided with the browser), so these classes simply will not work. Second (and more importantly), this solution is not as portable as one written in straight Java. While libpq is portable to all major flavors of Unix, and the code we'll write will be as well, there is currently no libpq for Windows95/NT or the Mac.

Apart from being simpler, there is one other advantage to writing this in native code: When the Postgres95 project releases bug fixes or changes their communication protocol, little or no change will be required to our code.

Sidebar: How to get Postgres and Java

We will proceed in three steps, providing examples of how to use each part along the way.

First, we'll create wrappers for libpq's PGconn, and PGResult. This will allow us to connect to the database, issue queries, and process the results.

Then, we'll write a new interface to Postgres95's Large Objects (or blobs in other databases), using Java's Stream classes.

Finally, we'll use Java's threads to provide an easy, behind the scenes interface to Postgres95's asynchronous notification system.