| <html> |
| <body> |
| <p> |
| Provides the system's {@link java.lang.ref.ReferenceQueue} implementation as well as |
| different forms of reference objects which impose special behavior on the |
| garbage collector. The behavior depends on the type of {@link java.lang.ref.Reference} |
| being used. Three different type of references exist, each being weaker |
| than the preceding one: {@link java.lang.ref.SoftReference}, {@link java.lang.ref.WeakReference}, and |
| {@link java.lang.ref.PhantomReference}. "Weakness" here means that less restrictions are |
| being imposed on the garbage collector as to when it is allowed to |
| actually garbage-collect the referenced object. |
| </p> |
| <p> |
| In order to use reference objects properly it is important to understand |
| the different types of reachability that trigger their clearing and |
| enqueueing. The following table lists these, from strongest to weakest. |
| For each row, an object is said to have the reachability on the left side |
| if (and only if) it fulfills all of the requirements on the right side. In |
| all rows, consider the <em>root set</em> to be a set of references that |
| are "resistant" to garbage collection (that is, running threads, method |
| parameters, local variables, static fields and the like). |
| </p> |
| <p> |
| <a name="definitions"></a> |
| <table> |
| <tr> |
| <td> |
| Strongly reachable |
| </td> |
| <td> |
| <ul> |
| <li> |
| There exists at least one path from the root set to the object |
| that does not traverse any instance of a {@link java.lang.ref.Reference} |
| subclass. |
| </li> |
| </ul> |
| </td> |
| </tr> |
| |
| <tr> |
| <td> |
| Softly reachable |
| </td> |
| <td> |
| <ul> |
| <li> |
| The object is not strongly reachable. |
| </li> |
| <li> |
| There exists at least one path from the root set to the object |
| that does traverse a {@link java.lang.ref.SoftReference} instance, but no |
| {@link java.lang.ref.WeakReference} or {@link java.lang.ref.PhantomReference} instances. |
| </li> |
| </ul> |
| </td> |
| </tr> |
| |
| <tr> |
| <td> |
| Weakly reachable |
| </td> |
| <td> |
| <ul> |
| <li> |
| The object is neither strongly nor softly reachable. |
| </li> |
| <li> |
| There exists at least one path from the root set to the object |
| that does traverse a {@link java.lang.ref.WeakReference} instance, but no |
| {@link java.lang.ref.PhantomReference} instances. |
| </li> |
| </ul> |
| </td> |
| </tr> |
| |
| <tr> |
| <td> |
| Phantom-reachable |
| </td> |
| <td> |
| <ul> |
| <li> |
| The object is neither strongly, softly nor weakly reachable. |
| </li> |
| <li> |
| The object is referenced by a {@link java.lang.ref.PhantomReference} instance. |
| </li> |
| <li> |
| The object has already been finalized. |
| </li> |
| </ul> |
| </td> |
| </tr> |
| </table> |
| </p> |
| </body> |
| </html> |
