Make the TemplateProcessingTracer interface slightly more useful.

src/main/java/freemarker/core/Environment.java

@@ -302,13 +302,6 @@ private void clearCachedValues() {
         cachedURLEscapingCharsetSet = false;
     }
 
-    /**
-     * Sets the tracer to use for this environment.
-     */
-    public void setTracer(TemplateProcessingTracer tracer) {
-        currentTracer = tracer;
-    }
-
     /**
      * Processes the template to which this environment belongs to.
      */
@@ -320,14 +313,12 @@ public void process() throws TemplateException, IOException {
             clearCachedValues();
             try {
                 doAutoImportsAndIncludes(this);
-                if (currentTracer != null) currentTracer.start();
                 visit(getTemplate().getRootTreeNode());
                 // It's here as we must not flush if there was an exception.
                 if (getAutoFlush()) {
                     out.flush();
                 }
             } finally {
-                if (currentTracer != null) currentTracer.end();
                 // It's just to allow the GC to free memory...
                 clearCachedValues();
             }
@@ -2882,6 +2873,13 @@ public TemplateModel get(String key) throws TemplateModelException {
         };
     }
 
+    /**
+     * Sets the tracer to use for this environment.
+     */
+    public void setTracer(TemplateProcessingTracer tracer) {
+        currentTracer = tracer;
+    }
+
     private void pushElement(TemplateElement element) {
         final int newSize = ++instructionStackSize;
         TemplateElement[] instructionStack = this.instructionStack;
@@ -2895,12 +2893,19 @@ private void pushElement(TemplateElement element) {
         }
         instructionStack[newSize - 1] = element;
         if (currentTracer != null) {
-            currentTracer.trace(element.getTemplate(), element.getBeginColumn(), element.getBeginLine(),
+            currentTracer.enterElement(element.getTemplate(),
+                    element.getBeginColumn(), element.getBeginLine(),
                     element.getEndColumn(), element.getEndLine(), element.isLeaf());
         }
     }
 
     private void popElement() {
+        if (currentTracer != null) {
+            TemplateElement element = instructionStack[instructionStackSize - 1];
+            currentTracer.exitElement(element.getTemplate(),
+                    element.getBeginColumn(), element.getBeginLine(),
+                    element.getEndColumn(), element.getEndLine());
+        }
         instructionStackSize--;
     }
 

src/main/java/freemarker/core/TemplateProcessingTracer.java

@@ -36,28 +36,21 @@
  */
 public interface TemplateProcessingTracer {
 
-    /**
-     * Invoked when {@link Template.process()} starts processing the template.
-     * 
-     * @since 2.3.23
-     */
-    void start();
-
     /**
      * Invoked by {@link Environment} whenever it starts processing a new template element. {@code
      * isLeafElement} indicates whether this element is a leaf, or whether the tracer should expect
      * to receive lower-level elements within the context of this one.
      * 
      * @since 2.3.23
      */
-    void trace(Template template, int beginColumn, int beginLine, int endColumn, int endLine,
+    void enterElement(Template template, int beginColumn, int beginLine, int endColumn, int endLine,
             boolean isLeafElement);
 
     /**
-     * Invoked when template processing is finished.
+     * Invoked by {@link Environment} whenever it completes processing a new template element.
      * 
      * @since 2.3.23
      */
-    void end();
+    void exitElement(Template template, int beginColumn, int beginLine, int endColumn, int endLine);
 
 }

src/test/java/freemarker/core/TemplateProcessingTracerTest.java

@@ -41,7 +41,7 @@ public class TemplateProcessingTracerTest {
             "Nope.\n" +
             "</#list>\n" +
             "<#list [] as item>\n" +
-            "${item}<#else>\n" +
+            "${item}<#else>" +
             "Yup.\n" +
             "</#list>\n";
 
@@ -50,31 +50,34 @@ public void test() throws Exception {
         Configuration cfg = new Configuration(Configuration.VERSION_2_3_32);
         Template t = new Template("test.ftl", TEMPLATE_TEXT, cfg);
         StringWriter sw = new StringWriter();
-        Tracer tracer = new Tracer();
+        Tracer tracer = new Tracer(TEMPLATE_TEXT);
         Environment env = t.createProcessingEnvironment(null, sw);
         env.setTracer(tracer);
         env.process();
 
-        List<Integer> expected = Arrays.asList(2, 3, 5, 5, 5, 9);
-        assertEquals(expected, tracer.linesVisited);
+        List<String> expected = Arrays.asList("Yup.", "Always.", "${item}", "${item}", "${item}", "Yup.");
+        assertEquals(expected, tracer.elementsVisited);
     }
 
     private static class Tracer implements TemplateProcessingTracer {
-        ArrayList<Integer> linesVisited;
+        final ArrayList<String> elementsVisited;
+        final String[] templateLines;
 
-        Tracer() {
-            linesVisited = new ArrayList<>();
+        Tracer(String template) {
+            elementsVisited = new ArrayList<>();
+            templateLines = template.split("\\n");
         }
 
-        public void start() {}
-        public void end() {}
-
-        public void trace(Template template, int beginColumn, int beginLine, int endColumn, int endLine,
+        public void enterElement(Template template, int beginColumn, int beginLine, int endColumn, int endLine,
             boolean isLeafElement) {
             if (isLeafElement) {
-                linesVisited.add(beginLine);
+                String line = templateLines[beginLine - 1];
+                String elementText = line.substring(beginColumn - 1,
+                        endLine == beginLine ? Math.min(endColumn, line.length()) : line.length());
+                elementsVisited.add(elementText);
             }
         }
 
+        public void exitElement(Template template, int beginColumn, int beginLine, int endColumn, int endLine) {}
     }
 }