add testcase

src/main/java/com/alibaba/druid/util/Histogram.java

@@ -77,6 +77,10 @@ public class Histogram {
         return ranges;
     }
     
+    public long getValue(int index) {
+        return rangeCounters[index].get();
+    }
+    
     public String toString() {
     	StringBuilder buf = new StringBuilder();
     	buf.append('[');

src/test/java/com/alibaba/druid/bvt/filter/PoolStatFilterTest.java

@@ -0,0 +1,43 @@
+package com.alibaba.druid.bvt.filter;
+
+import java.sql.Connection;
+
+import junit.framework.Assert;
+import junit.framework.TestCase;
+
+import com.alibaba.druid.pool.DruidDataSource;
+import com.alibaba.druid.util.Histogram;
+import com.alibaba.druid.util.JdbcUtils;
+
+public class PoolStatFilterTest extends TestCase {
+
+    private DruidDataSource dataSource;
+
+    protected void setUp() throws Exception {
+        dataSource = new DruidDataSource();
+
+        dataSource.setUrl("jdbc:mock:xxx");
+        dataSource.setFilters("stat");
+        
+        dataSource.init();
+    }
+
+    protected void tearDown() throws Exception {
+        JdbcUtils.close(dataSource);
+    }
+
+    public void test_stat() throws Exception {
+        
+        Assert.assertTrue(dataSource.isInited());
+        
+        Histogram histogram = dataSource.getDataSourceStat().getConnectionHoldHistogram();
+        
+        Assert.assertEquals(0, histogram.getValue(0));
+
+        Connection conn = dataSource.getConnection();
+
+        conn.close();
+
+        Assert.assertEquals(1, histogram.getValue(0));
+    }
+}