No Silver Bullet - Garbage Collection for Java in Embedded Systems

Preventive Measures

The heap in an embedded system can't be expanded because of the lack of virtual memory. This makes running out of memory a critical situation that applications should avoid using preventive measures. Since it's impossible for a Java application to explicitly free memory (it can only make its best effort to ensure that references to an object are suppressed), it can be worthwhile to encourage references to huge but easily recomputable objects using weak references.

Weak references were introduced in JDK 1.2. They allow references to an object to be maintained without preventing the object from being reclaimed by the collector. Applications may also use the MemoryAdvice interface to be implemented by the RunTime class in JDK 1.2. It defines the getMemoryAdvice method that returns four indicators of the memory usage, as estimated by the runtime environment, from GREEN (everything is fine) to RED (take every conceivable action to discard objects).

In the case of an application running out of memory, an aggressive garbage collection pass should be performed. The garbage collector must be guaranteed the memory this operation requires. Since some stack and some heap may be necessary, the system needs to monitor the low-water mark, and prevent the application from totally running out of memory before taking action. If no memory can still be found, the system throws an OutOfMemoryError error and it's up to the application to selectively choose which parts should be discarded.