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From ctubbsii <...@git.apache.org>
Subject [GitHub] accumulo pull request #292: ACCUMULO-4681 Created Table & Namespace WeakRefe...
Date Mon, 14 Aug 2017 21:38:48 GMT
Github user ctubbsii commented on a diff in the pull request:

    https://github.com/apache/accumulo/pull/292#discussion_r133068592
  
    --- Diff: core/src/main/java/org/apache/accumulo/core/client/impl/Table.java ---
    @@ -16,24 +16,59 @@
      */
     package org.apache.accumulo.core.client.impl;
     
    +import java.lang.ref.WeakReference;
    +import java.util.WeakHashMap;
    +
     import org.apache.accumulo.core.client.Instance;
     
     public class Table {
     
       /**
        * Object representing an internal table ID. This class was created to help with type
safety. For help obtaining the value of a table ID from Zookeeper, see
        * {@link Tables#getTableId(Instance, String)}
    +   *
    +   * Uses an internal WeakHashMap and private constructor for storing a WeakReference
of every Table.ID. Therefore, a Table.ID can't be instantiated outside
    +   * this class and is accessed by calling Table.ID.{@link #of(String)}.
        */
       public static class ID extends AbstractId {
         private static final long serialVersionUID = 7399913185860577809L;
    +    static final WeakHashMap<String,WeakReference<Table.ID>> tableIds = new
WeakHashMap<>();
     
    -    public static final ID METADATA = new ID("!0");
    -    public static final ID REPLICATION = new ID("+rep");
    -    public static final ID ROOT = new ID("+r");
    +    public static final ID METADATA = of("!0");
    +    public static final ID REPLICATION = of("+rep");
    +    public static final ID ROOT = of("+r");
     
    -    public ID(final String canonical) {
    +    private ID(final String canonical) {
           super(canonical);
         }
    +
    +    /**
    +     * Get a Table.ID object for the provided canonical string.
    +     *
    +     * @param canonical
    +     *          table ID string
    +     * @return Table.ID object
    +     */
    +    public static Table.ID of(final String canonical) {
    +      return dedupeTableId(canonical);
    +    }
    +
    +    private static Table.ID dedupeTableId(String tableIdString) {
    +      Table.ID tableId;
    +      synchronized (tableIds) {
    +        WeakReference<Table.ID> tableIdRef = tableIds.get(tableIdString);
    +        if (tableIdRef != null) {
    +          tableId = tableIdRef.get();
    +          if (tableId != null) {
    +            return tableId;
    +          }
    +        }
    +
    +        tableId = new ID(tableIdString);
    +        tableIds.put(tableIdString, new WeakReference<>(tableId));
    --- End diff --
    
    Talked to @keith-turner about this briefly. I think the `WeakHashMap<String,WeakReference<>>`
is the right type, but the String that's used as a key should be a new one, so its presence
in the map is not dependent on the caller's strong reference to the key, but on the caller's
strong reference to the returned Table.ID object. In other words, it should be:
    
    ```java
        T ret;
        WeakReference<T> ref = cache.get(canonical);
        if (ref == null || (ret = ref.get()) == null) {
            String s = new String(canonical);
            cache.put(s, ret = new Table.ID(s));
        }
        return ret;
    ```
    
    That way, the new String is only contained inside the ID and its presence in the map is
contingent on the ID object still having a strong reference to it. Of course, the user could
call `ID.getCanonical()` and get a strong reference to the key, thus holding it in the map
even when the ID object is gone. This could be avoided by the constructor creating a second
copy... the first is held by the ID only for the purposes of having a strong reference to
the map key, and the second would be returned in `.getCanonical()` and `.toString()`.


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