When System property is empty use def value.

Motivation:

When system property is empty, the default value should be used.

Modification:

- Correctly use the default value in all cases
- Add unit tests

Result:

Correct behaviour

common/src/main/java/io/netty/util/internal/SystemPropertyUtil.java

@@ -103,7 +103,7 @@ public final class SystemPropertyUtil {
 
         value = value.trim().toLowerCase();
         if (value.isEmpty()) {
-            return true;
+            return def;
         }
 
         if ("true".equals(value) || "yes".equals(value) || "1".equals(value)) {

common/src/test/java/io/netty/util/internal/SystemPropertyUtilTest.java

@@ -0,0 +1,128 @@
+/*
+ * Copyright 2017 The Netty Project
+ *
+ * The Netty Project licenses this file to you 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.netty.util.internal;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class SystemPropertyUtilTest {
+
+    @Before
+    public void clearSystemPropertyBeforeEach() {
+        System.clearProperty("key");
+    }
+
+    @Test(expected = NullPointerException.class)
+    public void testGetWithKeyNull() {
+        SystemPropertyUtil.get(null, null);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testGetWithKeyEmpty() {
+        SystemPropertyUtil.get("", null);
+    }
+
+    @Test
+    public void testGetDefaultValueWithPropertyNull() {
+        assertEquals("default", SystemPropertyUtil.get("key", "default"));
+    }
+
+    @Test
+    public void testGetPropertyValue() {
+        System.setProperty("key", "value");
+        assertEquals("value", SystemPropertyUtil.get("key"));
+    }
+
+    @Test
+    public void testGetBooleanDefaultValueWithPropertyNull() {
+        assertTrue(SystemPropertyUtil.getBoolean("key", true));
+        assertFalse(SystemPropertyUtil.getBoolean("key", false));
+    }
+
+    @Test
+    public void testGetBooleanDefaultValueWithEmptyString() {
+        System.setProperty("key", "");
+        assertTrue(SystemPropertyUtil.getBoolean("key", true));
+        assertFalse(SystemPropertyUtil.getBoolean("key", false));
+    }
+
+    @Test
+    public void testGetBooleanWithTrueValue() {
+        System.setProperty("key", "true");
+        assertTrue(SystemPropertyUtil.getBoolean("key", false));
+        System.setProperty("key", "yes");
+        assertTrue(SystemPropertyUtil.getBoolean("key", false));
+        System.setProperty("key", "1");
+        assertTrue(SystemPropertyUtil.getBoolean("key", true));
+    }
+
+    @Test
+    public void testGetBooleanWithFalseValue() {
+        System.setProperty("key", "false");
+        assertFalse(SystemPropertyUtil.getBoolean("key", true));
+        System.setProperty("key", "no");
+        assertFalse(SystemPropertyUtil.getBoolean("key", false));
+        System.setProperty("key", "0");
+        assertFalse(SystemPropertyUtil.getBoolean("key", true));
+    }
+
+    @Test
+    public void testGetBooleanDefaultValueWithWrongValue() {
+        System.setProperty("key", "abc");
+        assertTrue(SystemPropertyUtil.getBoolean("key", true));
+        System.setProperty("key", "123");
+        assertFalse(SystemPropertyUtil.getBoolean("key", false));
+    }
+
+    @Test
+    public void getIntDefaultValueWithPropertyNull() {
+        assertEquals(1, SystemPropertyUtil.getInt("key", 1));
+    }
+
+    @Test
+    public void getIntWithPropertValueIsInt() {
+        System.setProperty("key", "123");
+        assertEquals(123, SystemPropertyUtil.getInt("key", 1));
+    }
+
+    @Test
+    public void getIntDefaultValueWithPropertValueIsNotInt() {
+        System.setProperty("key", "NotInt");
+        assertEquals(1, SystemPropertyUtil.getInt("key", 1));
+    }
+
+    @Test
+    public void getLongDefaultValueWithPropertyNull() {
+        assertEquals(1, SystemPropertyUtil.getLong("key", 1));
+    }
+
+    @Test
+    public void getLongWithPropertValueIsLong() {
+        System.setProperty("key", "123");
+        assertEquals(123, SystemPropertyUtil.getLong("key", 1));
+    }
+
+    @Test
+    public void getLongDefaultValueWithPropertValueIsNotLong() {
+        System.setProperty("key", "NotInt");
+        assertEquals(1, SystemPropertyUtil.getLong("key", 1));
+    }
+
+}