BAEL-3439 - Add selector.wakeup() section (#10398)

* Add selector.wakeup() section

* Update SelectorTest.java

* Update core-java-modules/core-java-nio-2/src/test/java/com/baeldung/selector/SelectorTest.java

* Update SelectorTest.java

* Update and rename SelectorTest.java to SelectorUnitTest.java

* Update SelectorUnitTest.java

* Update and rename SelectorUnitTest.java to SelectorManualTest.java

core-java-modules/core-java-nio-2/pom.xml

@@ -14,5 +14,12 @@
         <version>0.0.1-SNAPSHOT</version>
         <relativePath>../</relativePath>
     </parent>
-
+    <dependencies>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>3.6.1</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
 </project>

core-java-modules/core-java-nio-2/src/test/java/com/baeldung/selector/SelectorManualTest.java

@@ -0,0 +1,64 @@
+package com.baeldung.selector;
+
+import org.junit.Test;
+import java.io.IOException;
+import java.nio.channels.Pipe;
+import java.nio.channels.SelectableChannel;
+import java.nio.channels.Selector;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import static java.nio.channels.SelectionKey.OP_READ;
+
+public class SelectorManualTest {
+
+    @Test
+    public void whenWakeUpCalledOnSelector_thenBlockedThreadReturns() throws IOException, InterruptedException {
+        Pipe pipe = Pipe.open();
+
+        Selector selector = Selector.open();
+        SelectableChannel channel = pipe.source();
+        channel.configureBlocking(false);
+        channel.register(selector, OP_READ);
+
+        List<String> invocationStepsTracker = Collections.synchronizedList(new ArrayList<>());
+
+        CountDownLatch latch = new CountDownLatch(1);
+
+        Thread thread = new Thread(() -> {
+            invocationStepsTracker.add(">> Count down");
+            latch.countDown();
+            try {
+                invocationStepsTracker.add(">> Start select");
+                selector.select();
+                invocationStepsTracker.add(">> End select");
+            } catch (IOException e) {
+                e.printStackTrace();
+            }
+        });
+
+        invocationStepsTracker.add(">> Start await");
+        thread.start();
+        latch.await();
+        invocationStepsTracker.add(">> End await");
+
+        invocationStepsTracker.add(">> Wakeup thread");
+
+        selector.wakeup();
+        channel.close();
+
+        assertThat(invocationStepsTracker)
+          .containsExactly(
+            ">> Start await",
+            ">> Count down",
+            ">> Start select",
+            ">> End await",
+            ">> Wakeup thread",
+            ">> End select"
+          );
+    }
+}