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org.vmmagic.pragma
Annotation Type Unpreemptible

@Retention(value=RUNTIME)
@Target(value={TYPE,METHOD})
public @interface Unpreemptible

By default all Java code is interruptible, that is scheduling or garbage collection may occur at points within the code. Code can be marked as Unpreemptible or Uninterruptible, that instructs the JVM to avoid garbage collection and thread scheduling. The Uninterruptible annotation disallows any operation that may cause garbage collection or thread scheduling, for example memory allocation. The Unpreemptible annotation doesn't disallow operations that can cause garbage collection or scheduling, but instructs the JVM to avoid inserting such operations during a block of code.

In the internals of a VM most code wants to be Uninterruptible. However, code involved in scheduling and locking will cause context switches, and creating exception objects may trigger garbage collection, this code is therefore Unpreemptible.

Any method that is marked as unpreemptible is treated specially by the machine code compiler:

  1. the normal thread switch test that would be emitted in the method prologue is omitted.
  2. the stack overflow test that would be emitted in the method prologue is omitted.
  3. calls to preemptible code causes warnings.

      4. This is the inverse of Preemptible.

      Optional Element Summary
       String value
                 
       

      value

      public abstract String value
      Returns:
      Explanation of why code needs to be unpreemptible
      Default:
      ""
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