Github user ctubbsii commented on a diff in the pull request:

    https://github.com/apache/accumulo/pull/292#discussion_r132825421
  
    --- 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 --
    
    To clarify what I mean, "put" doesn't replace a key if it already exists. 
It only updates the value. This also might be mitigated by using Table.ID as 
the key, since it's less likely that the Table.ID will exist as a key in the 
map and point to an empty WeakReference.


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