Merge pull request eugenp#5940 from JonCook/master

BAEL-2175 - Verbose Garbage Collection in Java

Author: lor6

core-java/src/main/java/com/baeldung/gc/VerboseGarbageCollectorRunner.java

@@ -0,0 +1,63 @@
+package com.baeldung.gc;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * A simple Java program to demonstrate how to enable verbose Garbage Collection (GC) logging.
+ * <p>
+ * This simple program loads 3 million {@link java.lang.String} instances into a {@link java.util.HashMap} 
+ * object before making an explicit call to the garbage collector using <code>System.gc()</code>.
+ * <p>
+ * Finally, it removes 2 million of the {@link java.lang.String} instances from the {@link java.util.HashMap}. 
+ * We also explicitly use <code>System.out.println</code> to make interpreting the output easier.
+ * <p>
+ * Run this program with the following arguments to see verbose GC logging in its complete form:
+ * <pre>
+ * -XX:+UseSerialGC -Xms1024m -Xmx1024m -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintGCDateStamps -Xloggc:/path/to/file/gc.log
+ * </pre>
+ * Where:
+ * <ul>
+ *   <li><code>-XX:+UseSerialGC</code> - specify the serial garbage collector.</li>
+ *   <li><code>-Xms1024m</code> - specify the minimal heap size.</li>
+ *   <li><code>-Xmx1024m</code> - specify the maximal heap size.</li>
+ *   <li><code>-verbose:gc</code> - activate the logging of garbage collection information in its simplest form.</li>
+ *   <li><code>-XX:+PrintGCDetails</code> - activate detailed information about garbage collection.</li>
+ *   <li><code>-XX:+PrintGCTimeStamps</code> - include a timestamp in the output reflecting the real-time passed in seconds since the JVM started.</li>
+ *   <li><code>-XX:+PrintGCDateStamps</code> - include the absolute date and time at the start of each line.</li>
+ *   <li><code>-Xloggc</code> - by default the GC log is written to stdout. Specify an output file via this argument.</li>
+ * </ul>
+ * <p>
+ * It should be noted that the first three arguments are not strictly necessary but for the purposes or the example
+ * help really simplify things.
+ *
+ */
+public class VerboseGarbageCollectorRunner {
+
+    private static Map<String, String> stringContainer = new HashMap<>();
+
+    public static void main(String[] args) {
+        System.out.println("Start of program!");
+        String stringWithPrefix = "stringWithPrefix";
+
+        // Load Java Heap with 3 M java.lang.String instances
+        for (int i = 0; i < 3000000; i++) {
+            String newString = stringWithPrefix + i;
+            stringContainer.put(newString, newString);
+        }
+        System.out.println("MAP size: " + stringContainer.size());
+
+        // Explicit GC!
+        System.gc();
+
+        // Remove 2 M out of 3 M
+        for (int i = 0; i < 2000000; i++) {
+            String newString = stringWithPrefix + i;
+            stringContainer.remove(newString);
+        }
+
+        System.out.println("MAP size: " + stringContainer.size());
+        System.out.println("End of program!");
+    }
+
+}