Add "override" option to preserve existing properties from being over…

…ridden

This fixes #21

src/main/java/org/codehaus/mojo/properties/ReadPropertiesMojo.java

@@ -130,6 +130,19 @@ public void setKeyPrefix( String keyPrefix )
     @Parameter( defaultValue = "false" )
     private boolean useDefaultValues;
 
+    /**
+     * Determine, whether existing properties should be overridden or not. Default: <code>true</true>.
+     *
+     * @since 1.2.0
+     */
+    @Parameter( defaultValue = "true" )
+    private boolean override = true;
+
+    public void setOverride( boolean override )
+    {
+        this.override = override;
+    }
+
     /**
      * Used for resolving property placeholders.
      */
@@ -202,19 +215,23 @@ private void loadProperties( Resource resource )
 
             try ( InputStream stream = resource.getInputStream() )
             {
+                String effectivePrefix = "";
                 if ( keyPrefix != null )
                 {
-                    Properties properties = new Properties();
-                    properties.load( stream );
-                    Properties projectProperties = project.getProperties();
-                    for ( String key : properties.stringPropertyNames() )
-                    {
-                        projectProperties.put( keyPrefix + key, properties.get( key ) );
-                    }
+                    effectivePrefix = keyPrefix;
                 }
-                else
+
+                Properties properties = new Properties();
+                properties.load( stream );
+                Properties projectProperties = project.getProperties();
+
+                for( String key: properties.stringPropertyNames() )
                 {
-                    project.getProperties().load( stream );
+                    String propertyName = effectivePrefix + key;
+                    if ( override || !projectProperties.containsKey( propertyName ) )
+                    {
+                        projectProperties.put( propertyName, properties.get( key ) );
+                    }
                 }
             }
         }

src/test/java/org/codehaus/mojo/properties/ReadPropertiesMojoTest.java

@@ -152,6 +152,61 @@ public void testDefaultValueForUnresolvedPropertyWithEnabledFlag() throws MojoFa
         assertEquals("  ", value13);
     }
 
+    @Test
+    public void readPropertiesOverridingExisting() throws Exception {
+
+    	File testPropertyFile = getPropertyFileForTesting();
+    	// load properties directly for comparison later
+    	Properties testProperties = new Properties();
+    	testProperties.load(new FileReader(testPropertyFile));
+
+    	// Existing property value should be overridden
+    	projectStub.getProperties().put("test.property2", "old-value");
+
+    	// do the work
+    	readPropertiesMojo.setFiles(new File[]{testPropertyFile});
+    	readPropertiesMojo.execute();
+
+    	// check results
+    	Properties projectProperties = projectStub.getProperties();
+    	assertNotNull(projectProperties);
+    	// it should not be empty
+    	assertNotEquals(0, projectProperties.size());
+
+    	// we are not adding prefix, so properties should be same as in file
+    	assertEquals(testProperties.size(), projectProperties.size());
+    	assertEquals(testProperties, projectProperties);
+
+    }
+
+    @Test
+    public void readPropertiesPreserveExisting() throws Exception {
+
+        File testPropertyFile = getPropertyFileForTesting();
+        // load properties directly for comparison later
+        Properties testProperties = new Properties();
+        testProperties.load(new FileReader(testPropertyFile));
+
+        // Existing property should keep the value
+        testProperties.put("test.property2", "old-value");
+    	projectStub.getProperties().put("test.property2", "old-value");
+
+        // do the work
+        readPropertiesMojo.setFiles(new File[]{testPropertyFile});
+        readPropertiesMojo.setOverride(false);
+        readPropertiesMojo.execute();
+
+        // check results
+        Properties projectProperties = projectStub.getProperties();
+        assertNotNull(projectProperties);
+        // it should not be empty
+        assertNotEquals(0, projectProperties.size());
+
+        // we are not adding prefix, so properties should be same as in file
+        assertEquals(testProperties.size(), projectProperties.size());
+        assertEquals(testProperties, projectProperties);
+    }
+
     /**
      * with the flag disabled (default behavior) nothing gets replaced
      * ':' is treated as a regular character and part of the property name