optimized memory used

src/test/java/com/alibaba/druid/stat/MemoryTest.java

@@ -0,0 +1,38 @@
+package com.alibaba.druid.stat;
+
+import java.lang.management.ManagementFactory;
+import java.text.NumberFormat;
+import java.util.concurrent.atomic.AtomicLong;
+
+import junit.framework.TestCase;
+
+public class MemoryTest extends TestCase {
+
+    public void test_0() throws Exception {
+        A item = new A();
+        gc();
+        final int COUNT = 1024 * 1024;
+        A[] items = new A[COUNT];
+
+        long memoryStart = ManagementFactory.getMemoryMXBean().getHeapMemoryUsage().getUsed();
+
+        for (int i = 0; i < COUNT; ++i) {
+            items[i] = new A();
+            // items[i] = Histogram.makeHistogram(20);
+        }
+
+        long memoryEnd = ManagementFactory.getMemoryMXBean().getHeapMemoryUsage().getUsed();
+
+        System.out.println("memory used : " + NumberFormat.getInstance().format(memoryEnd - memoryStart));
+    }
+
+    private void gc() {
+        for (int i = 0; i < 10; ++i) {
+            System.gc();
+        }
+    }
+
+    public static class A {
+        private volatile long v;
+    }
+}