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Technology in brief
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A function or trigger in SQL resolves to a static method in a Java class. In order for the function to execute, the appointed class must be installed in the database. PL/Java adds a set of functions that helps installing and maintaining the java classes. Classes are loaded into the database from normal Java archives (AKA jars). A Jar may optionally contain a deployment descriptor that in turn contains SQL commands to be executed when the jar is deployed/undeployed. The functions are modeled after the standards proposed for SQL 2003.
PL/Java implements a standardized way of passing parameters and return values. Complex types and sets are passed using the standard JDBC ResultSet class. Great care has been taken not to introduce any proprietary interfaces unless absolutely necessary so that Java code written using PL/Java becomes as database agnostic as possible.
A JDBC driver is included in PL/Java. This driver is written directly on top of the PostgreSQL internal SPI routines. This driver is essential since it's very common for functions and triggers to reuse the database. When they do, they must use the same transactional boundaries that where used by the caller.
PL/Java is optimized for performance. The Java virtual machine executes within the same process as the backend itself. This vouches for a very low call overhead. PL/Java is designed with the objective to enable the power of Java to the database itself so that database intensive business logic can execute as close to the actual data as possible.
The standard Java Native Interface (JNI) is used when bridging calls from the backend into the Java VM and vice versa. Please read the rationale behind The choice of JNI and a more in-depth discussion about some implementation details.
The versions of PostgreSQL and Java targeted by current PL/Java development can be reviewed on the versions page.