Real-Time Java™ Platform Programming

Rules

The JVM has to enforce certain rules to keep the new types of memory from breaking the garbage collector's commitment not to leave bad pointers. The underlying rule is that a reference variable cannot contain a reference to an object that can be freed before the reference variable. This rule is enforced by the JVM. Table 13-1 summarizes the assignment rules.

Table 13-1. Assignment rules

	Reference to Heap	Reference to Immortal	Reference to Scoped
Heap	Yes	Yes	No
Immortal	Yes	Yes	No
Scoped	Yes	Yes	“Available” scopes
Local variable	Yes	Yes	“Available” scopes

If it must, the JVM can enforce the assignment rules when it executes the bytecode instructions that store object references, but since that enforcement slows execution of the JVM, a ...