Class SharedObject

java.lang.Object
com.ibm.icu.impl.coll.SharedObject
All Implemented Interfaces:
Cloneable
Direct Known Subclasses:
CollationSettings

public class SharedObject extends Object implements Cloneable
Base class for shared, reference-counted, auto-deleted objects. Java subclasses are mutable and must implement clone().

In C++, the SharedObject base class is used for both memory and ownership management. In Java, memory management (deletion after last reference is gone) is up to the garbage collector, but the reference counter is still used to see whether the referent is the sole owner.

Usage:

 class S extends SharedObject {
     public clone() { ... }
 }

 // Either use the nest class Reference (which costs an extra allocation),
 // or duplicate its code in the class that uses S
 // (which duplicates code and is more error-prone).
 class U {
     // For read-only access, use s.readOnly().
     // For writable access, use S ownedS = s.copyOnWrite();
     private SharedObject.Reference<S> s;
     // Returns a writable version of s.
     // If there is exactly one owner, then s itself is returned.
     // If there are multiple owners, then s is replaced with a clone,
     // and that is returned.
     private S getOwnedS() {
         return s.copyOnWrite();
     }
     public U clone() {
         ...
         c.s = s.clone();
         ...
     }
 }

 class V {
     // For read-only access, use s directly.
     // For writable access, use S ownedS = getOwnedS();
     private S s;
     // Returns a writable version of s.
     // If there is exactly one owner, then s itself is returned.
     // If there are multiple owners, then s is replaced with a clone,
     // and that is returned.
     private S getOwnedS() {
         if(s.getRefCount() > 1) {
             S ownedS = s.clone();
             s.removeRef();
             s = ownedS;
             ownedS.addRef();
         }
         return s;
     }
     public U clone() {
         ...
         s.addRef();
         ...
     }
     protected void finalize() {
         ...
         if(s != null) {
             s.removeRef();
             s = null;
         }
         ...
     }
 }
 
Either use only Java memory management, or use addRef()/removeRef(). Sharing requires reference-counting. TODO: Consider making this more widely available inside ICU, or else adopting a different model.
  • Field Details

  • Constructor Details

    • SharedObject

      public SharedObject()
      Initializes refCount to 0.
  • Method Details

    • clone

      public SharedObject clone()
      Initializes refCount to 0.
      Overrides:
      clone in class Object
    • addRef

      public final void addRef()
      Increments the number of references to this object. Thread-safe.
    • removeRef

      public final void removeRef()
      Decrements the number of references to this object, and auto-deletes "this" if the number becomes 0. Thread-safe.
    • getRefCount

      public final int getRefCount()
      Returns the reference counter. Uses a memory barrier.
    • deleteIfZeroRefCount

      public final void deleteIfZeroRefCount()