@Override
  public boolean isEmpty() {
    return controller().invoke(() -> map().isEmpty());
  }
JUnit Rule that provides a shared distributed map in DistributedTest VMs.

<p>
{@code DistributedMap} can be used in DistributedTests as a {@code Rule}:

<pre>
{@literal @}Rule
public DistributedRule distributedRule = new DistributedRule();

{@literal @}Rule
public DistributedMap&lt;String, String&gt; distributedMap = new DistributedMap&lt;&gt;();

{@literal @}Test
public void accessDistributedMapInEveryVm() {
  distributedMap.put("key", "value);

  for (VM vm : getAllVMs()) {
    vm.invoke(() -&gt; {
      assertThat(distributedMap.get("key")).isEqualTo("value");
    });
  }
}
</pre>
geode-dunit/src/main/java/org/apache/geode/test/dunit/rules/DistributedMap.java