Added compiler option to provide defaults for missing values

Call missingVariableValue(value) on a Compiler object to get a new
version that will insert "value" for missing values (rather than
throwing an exception).  Useful when you have templates you would
rather generated *something* instead of erroring or for when objects may
have null values themselves.

src/main/java/com/samskivert/mustache/Mustache.java

@@ -38,6 +38,9 @@ public class Mustache
         /** Whether or not standards mode is enabled. */
         public final boolean standardsMode;
 
+        /** Replace missnig values with this value - if null will throw an exception on missing values. */
+        public final String missingVariableValue;
+
         /** The template loader in use during this compilation. */
         public final TemplateLoader loader;
 
@@ -53,24 +56,32 @@ public class Mustache
 
         /** Returns a compiler that either does or does not escape HTML by default. */
         public Compiler escapeHTML (boolean escapeHTML) {
-            return new Compiler(escapeHTML, this.standardsMode, this.loader);
+            return new Compiler(escapeHTML, this.standardsMode, this.missingVariableValue, this.loader);
         }
 
         /** Returns a compiler that either does or does not use standards mode. Standards mode
          * disables the non-standard JMustache extensions like looking up missing names in a parent
          * context. */
         public Compiler standardsMode (boolean standardsMode) {
-            return new Compiler(this.escapeHTML, standardsMode, this.loader);
+            return new Compiler(this.escapeHTML, standardsMode, this.missingVariableValue, this.loader);
+        }
+
+        /**
+         * Returns a compiler that will replace missing variables with the given value
+         **/
+        public Compiler missingVariableValue (String missingVariableValue) {
+            return new Compiler(this.escapeHTML, this.standardsMode, missingVariableValue, this.loader);
         }
 
         /** Returns a compiler configured to use the supplied template loader to handle partials. */
         public Compiler withLoader (TemplateLoader loader) {
-            return new Compiler(this.escapeHTML, this.standardsMode, loader);
+            return new Compiler(this.escapeHTML, this.standardsMode, this.missingVariableValue, loader);
         }
 
-        protected Compiler (boolean escapeHTML, boolean standardsMode, TemplateLoader loader) {
+        protected Compiler (boolean escapeHTML, boolean standardsMode, String missingVariableValue, TemplateLoader loader) {
             this.escapeHTML = escapeHTML;
             this.standardsMode = standardsMode;
+            this.missingVariableValue = missingVariableValue;
             this.loader = loader;
         }
     }
@@ -88,7 +99,7 @@ public class Mustache
      */
     public static Compiler compiler ()
     {
-        return new Compiler(true, false, FAILING_LOADER);
+        return new Compiler(true, false, null, FAILING_LOADER);
     }
 
     /**
@@ -468,8 +479,7 @@ public class Mustache
             _escapeHTML = escapeHTML;
         }
         @Override public void execute (Template tmpl, Template.Context ctx, Writer out)  {
-            Object value = tmpl.getValue(ctx, _name, _line);
-            // TODO: configurable behavior on missing values
+            Object value = tmpl.getValueWithDefault(ctx, _name, _line);
             if (value == null) {
                 throw new MustacheException(
                     "No key, method or field with name '" + _name + "' on line " + _line);

src/main/java/com/samskivert/mustache/Template.java

@@ -126,6 +126,12 @@ public class Template
         return null;
     }
 
+    protected Object getValueWithDefault (Context ctx, String name, int line)
+    {
+        Object value = getValue(ctx, name, line);
+        return value != null? value : _compiler.missingVariableValue;
+    }
+
     protected Object getValueIn (Object data, String name, int line)
     {
         if (data == null) {

src/test/java/com/samskivert/mustache/MustacheTest.java

@@ -284,6 +284,27 @@ public class MustacheTest
                             "parentProperty", "bar"));
     }
 
+    @Test(expected = MustacheException.class)
+    public void testMissingValue () {
+        String tmpl = "{{ missing }} {{ notmissing }}";
+        Mustache.compiler().compile(tmpl).
+            execute(Collections.singletonMap("notmissing", "bar"));
+    }
+
+    @Test public void testMissingValueWithDefault () {
+        String tmpl = "{{ missing }}{{ notmissing }}";
+        String result = Mustache.compiler().missingVariableValue("").compile(tmpl).
+            execute(Collections.singletonMap("notmissing", "bar"));
+        assertEquals("bar", result);
+    }
+
+    @Test public void testMissingValueWithDefaultNonEmptyString () {
+        String tmpl = "{{ missing }}{{ notmissing }}";
+        String result = Mustache.compiler().missingVariableValue("foo").compile(tmpl).
+            execute(Collections.singletonMap("notmissing", "bar"));
+        assertEquals("foobar", result);
+    }
+
     protected void test (Mustache.Compiler compiler, String expected, String template, Object ctx)
     {
         assertEquals(expected, compiler.compile(template).execute(ctx));