Handle null dynamic keys in lookup helper

handlebars/src/main/java/com/github/jknack/handlebars/helper/LookupHelper.java

@@ -36,7 +36,7 @@ public Object apply(final Object context, final Options options) throws IOExcept
     if (context == null) {
       return null;
     }
-    if (options.params.length <= 0) {
+    if (options.params.length <= 0 || options.param(0) == null) {
       return context;
     }
     Context ctx = Context.newBuilder(options.context, context).build();

handlebars/src/test/java/com/github/jknack/handlebars/LookupHelperTest.java

@@ -0,0 +1,132 @@
+/*
+ * Handlebars.java: https://github.com/jknack/handlebars.java
+ * Apache License Version 2.0 http://www.apache.org/licenses/LICENSE-2.0
+ * Copyright (c) 2012 Edgar Espina
+ */
+package com.github.jknack.handlebars;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+import java.io.IOException;
+import java.util.Arrays;
+
+import org.junit.jupiter.api.Test;
+
+import com.github.jknack.handlebars.helper.LookupHelper;
+
+class LookupHelperTest extends AbstractTest {
+
+  @Test
+  void nullKeyFallsBackToContext() throws IOException {
+    Hash context = new Hash().$("name", "fallback");
+    assertSame(context, lookup(context, (Object) null));
+  }
+
+  @Test
+  void nullKeyFallsBackToNonMapContext() throws IOException {
+    Object context = new Object();
+    assertSame(context, lookup(context, (Object) null));
+  }
+
+  @Test
+  void missingNestedDynamicKeyRendersFallback() throws IOException {
+    shouldCompileTo(
+        "{{#with (lookup object person.id)}}{{name}}{{/with}}",
+        new Hash().$("object", new Hash().$("name", "fallback")).$("person", new Hash()),
+        "fallback");
+  }
+
+  @Test
+  void explicitNullDynamicKeyRendersFallback() throws IOException {
+    shouldCompileTo(
+        "{{#with (lookup object key)}}{{name}}{{/with}}",
+        new Hash().$("object", new Hash().$("name", "fallback")).$("key", null),
+        "fallback");
+  }
+
+  @Test
+  void nullParentKeyFallsBackInsideEach() throws IOException {
+    shouldCompileTo(
+        "{{#each objects}}{{#with (lookup this ../key)}}{{name}};{{/with}}{{/each}}",
+        new Hash()
+            .$("objects", Arrays.asList(new Hash().$("name", "A"), new Hash().$("name", "B")))
+            .$("key", null),
+        "A;B;");
+  }
+
+  @Test
+  void nullKeyDoesNotLookUpTheStringNull() throws IOException {
+    Hash context = new Hash().$("name", "fallback").$("null", "literal");
+    assertSame(context, lookup(context, (Object) null));
+  }
+
+  @Test
+  void stringNullRemainsAPropertyName() throws IOException {
+    assertEquals("literal", lookup(new Hash().$("null", "literal"), "null"));
+  }
+
+  @Test
+  void nullContextRemainsNull() throws IOException {
+    assertNull(lookup(null, "name"));
+  }
+
+  @Test
+  void nullContextAndKeyRemainNull() throws IOException {
+    assertNull(lookup(null, (Object) null));
+  }
+
+  @Test
+  void missingParameterFallsBackToContext() throws IOException {
+    Hash context = new Hash().$("name", "fallback");
+    assertSame(context, lookup(context));
+  }
+
+  @Test
+  void missingPropertyFallsBackToContext() throws IOException {
+    Hash context = new Hash().$("name", "fallback");
+    assertSame(context, lookup(context, "absent"));
+  }
+
+  @Test
+  void nullPropertyFallsBackToContext() throws IOException {
+    Hash context = new Hash().$("value", null);
+    assertSame(context, lookup(context, "value"));
+  }
+
+  @Test
+  void falsePropertyIsReturned() throws IOException {
+    assertEquals(false, lookup(new Hash().$("value", false), "value"));
+  }
+
+  @Test
+  void zeroPropertyIsReturned() throws IOException {
+    assertEquals(0, lookup(new Hash().$("value", 0), "value"));
+  }
+
+  @Test
+  void emptyPropertyIsReturned() throws IOException {
+    assertEquals("", lookup(new Hash().$("value", ""), "value"));
+  }
+
+  @Test
+  void nestedPropertyIsReturned() throws IOException {
+    assertEquals(
+        "found", lookup(new Hash().$("person", new Hash().$("name", "found")), "person.name"));
+  }
+
+  private Object lookup(final Object context, final Object... params) throws IOException {
+    Handlebars handlebars = new Handlebars();
+    Options options =
+        new Options.Builder(
+                handlebars,
+                LookupHelper.NAME,
+                TagType.VAR,
+                Context.newContext(context),
+                handlebars.compileInline(""))
+            .setParams(params)
+            .build();
+    return LookupHelper.INSTANCE.apply(context, options);
+  }
+}