add UptimeGauge (#3)

README.md

@@ -26,6 +26,7 @@ A set of additional metrics complementing Dropwizards [metrics-jvm](https://gith
 ```java
 metrics.register("jvm.native", new NativeMemoryUsageGaugeSet());
 metrics.register("jvm.fds.count", new FileDescriptorCountGauge());
+metrics.register("jvm.uptime", new UptimeGauge());
 ```
 
 ## Available Metrics
@@ -49,6 +50,10 @@ The `NativeMemoryUsageGaugeSet` reads values from `/proc/self/smaps`.
 
 Provides the count of open file descriptors in use by the JVM process.
 
+### UptimeGauge
+
+Provides uptime in `ms`.
+
 ## Notes
 * procfs data is cached for `100ms` in order to relief the filesystem pressure
   when `Metric`s based on this data are queried by the registry one after

src/main/java/io/github/mweirauch/metrics/jvm/extras/UptimeGauge.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright © 2016 Michael Weirauch ([email protected])
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.github.mweirauch.metrics.jvm.extras;
+
+import java.lang.management.ManagementFactory;
+import java.lang.management.RuntimeMXBean;
+import java.util.Objects;
+
+import com.codahale.metrics.Gauge;
+
+public class UptimeGauge implements Gauge<Long> {
+
+    private final RuntimeMXBean runtimeBean;
+
+    public UptimeGauge() {
+        this(ManagementFactory.getRuntimeMXBean());
+    }
+
+    public UptimeGauge(RuntimeMXBean runtimeBean) {
+        this.runtimeBean = Objects.requireNonNull(runtimeBean);
+    }
+
+    @Override
+    public Long getValue() {
+        return runtimeBean.getUptime();
+    }
+
+}

src/test/java/io/github/mweirauch/metrics/jvm/extras/UptimeGaugeUnit0Test.java

@@ -0,0 +1,74 @@
+/*
+ * Copyright © 2016 Michael Weirauch ([email protected])
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.github.mweirauch.metrics.jvm.extras;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import java.lang.management.ManagementFactory;
+import java.lang.management.RuntimeMXBean;
+
+import org.junit.Test;
+
+import com.google.common.testing.NullPointerTester;
+import com.google.common.testing.NullPointerTester.Visibility;
+
+public class UptimeGaugeUnit0Test {
+
+    private final RuntimeMXBean runtimeBean = mock(RuntimeMXBean.class);
+
+    @Test
+    public void testNullContract() {
+        final UptimeGauge uut = new UptimeGauge(runtimeBean);
+
+        final NullPointerTester npt = new NullPointerTester();
+
+        npt.testConstructors(uut.getClass(), Visibility.PACKAGE);
+        npt.testStaticMethods(uut.getClass(), Visibility.PACKAGE);
+        npt.testInstanceMethods(uut, Visibility.PACKAGE);
+    }
+
+    @Test
+    public void testInstantiation() {
+        new UptimeGauge();
+    }
+
+    @Test
+    public void testGetValue() throws Exception {
+        final UptimeGauge uut = new UptimeGauge(runtimeBean);
+        when(runtimeBean.getUptime()).thenReturn(Long.valueOf(1337));
+
+        final Long value = uut.getValue();
+
+        assertNotNull(value);
+        assertEquals(Long.valueOf(1337), value);
+    }
+
+    @Test
+    public void testGetValueReal() throws Exception {
+        final UptimeGauge uut = new UptimeGauge(ManagementFactory.getRuntimeMXBean());
+        when(runtimeBean.getUptime()).thenReturn(Long.valueOf(1337));
+
+        final Long value = uut.getValue();
+
+        assertNotNull(value);
+        assertTrue(value > 0);
+    }
+
+}