Add a variable function to map from a variable to a value at runtime.

src/main/java/com/hubspot/jinjava/Jinjava.java

@@ -302,6 +302,7 @@ private Context copyGlobalContext() {
     globalContext.getAllFilters().forEach(context::registerFilter);
     globalContext.getAllFunctions().forEach(context::registerFunction);
     globalContext.getAllTags().forEach(context::registerTag);
+    context.setVariableFunction(globalContext.getVariableFunction());
     return context;
   }
 }

src/main/java/com/hubspot/jinjava/interpret/Context.java

@@ -45,6 +45,7 @@
 import java.util.Optional;
 import java.util.Set;
 import java.util.Stack;
+import java.util.function.Function;
 import java.util.stream.Collectors;
 
 public class Context extends ScopeMap<String, Object> {
@@ -102,6 +103,7 @@ public enum Library {
   private boolean validationMode = false;
   private boolean deferredExecutionMode = false;
   private boolean hideInterpreterErrors = false;
+  private Function<String, Object> variableFunction = null;
 
   public Context() {
     this(null, null, null);
@@ -172,6 +174,7 @@ public Context(
       new FunctionLibrary(parent == null, disabled.get(Library.FUNCTION));
     if (parent != null) {
       this.expressionStrategy = parent.expressionStrategy;
+      this.variableFunction = parent.variableFunction;
     }
   }
 
@@ -499,6 +502,14 @@ public void registerTag(Tag t) {
     tagLibrary.addTag(t);
   }
 
+  public Function<String, Object> getVariableFunction() {
+    return variableFunction;
+  }
+
+  public void setVariableFunction(final Function<String, Object> variableFunction) {
+    this.variableFunction = variableFunction;
+  }
+
   public ExpressionStrategy getExpressionStrategy() {
     return expressionStrategy;
   }

src/main/java/com/hubspot/jinjava/interpret/JinjavaInterpreter.java

@@ -412,6 +412,9 @@ public Object retraceVariable(String variable, int lineNumber, int startPosition
     Variable var = new Variable(this, variable);
     String varName = var.getName();
     Object obj = context.get(varName);
+    if (obj == null && context.getVariableFunction() != null) {
+      obj = context.getVariableFunction().apply(varName);
+    }
     if (obj != null) {
       if (obj instanceof DeferredValue) {
         throw new DeferredValueException(variable, lineNumber, startPosition);

src/test/java/com/hubspot/jinjava/interpret/VariableFunctionTest.java

@@ -0,0 +1,51 @@
+package com.hubspot.jinjava.interpret;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.hubspot.jinjava.Jinjava;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.function.Function;
+import org.junit.Test;
+
+public class VariableFunctionTest {
+  private static final Function<String, Object> VARIABLE_FUNCTION = s -> {
+    switch (s) {
+      case "name":
+        return "Jared";
+      case "title":
+        return "Mr.";
+      case "surname":
+        return "Stehler";
+      default:
+        return null;
+    }
+  };
+
+  @Test
+  public void willUseTheFunctionToPopulateVariables() {
+    final Jinjava jinjava = new Jinjava();
+    jinjava.getGlobalContext().setVariableFunction(VARIABLE_FUNCTION);
+    final Map<String, Object> context = new HashMap<>();
+
+    final String template = "<div>Hello, {{ title }} {{ name }} {{ surname }}!</div>";
+
+    final String renderedTemplate = jinjava.render(template, context);
+
+    assertThat(renderedTemplate).isEqualTo("<div>Hello, Mr. Jared Stehler!</div>");
+  }
+
+  @Test
+  public void willPreferTheContextOverTheFunctionToPopulateVariables() {
+    final Jinjava jinjava = new Jinjava();
+    jinjava.getGlobalContext().setVariableFunction(VARIABLE_FUNCTION);
+    final Map<String, Object> context = new HashMap<>();
+    context.put("name", "Greg");
+
+    final String template = "<div>Hello, {{ title }} {{ name }} {{ surname }}!</div>";
+
+    final String renderedTemplate = jinjava.render(template, context);
+
+    assertThat(renderedTemplate).isEqualTo("<div>Hello, Mr. Greg Stehler!</div>");
+  }
+}