In this section you will learn how to interact with the Context Stack in Handlebars.java.

The Context Stack

The Context class represent the context stack in Handlebars.java. You rarely need to instantiate aContextobject, bc Handlebars.java does it for you every time you call:

  String output = template.apply(model);
  // or
  template.apply(model, writer);

Theapplymethod accepts aContext or anObject. Theapply(Object)function, automatically convert the object into aContext.

Resolving values

  while (context != null && context.get("value") == null) {
    context = context.parent;
  }

A value will be resolved in the current context.

If the value isn't here, the parent context need to resolved the value.

If the value isn't in the whole stack, the value is resolved to null and nothing will be rendered.

Extending the context stack

A common use case in web applications is to access to the logged user from a web page.

  User user = ...;
  Object model = ...;
  Map modelWithUser = new HashMap();
  modelWithUser.put("user", user);
  modelWithUser.put("model", model);

  template.apply(modelWithUser);

This isn't so bad but there are at least two drawbacks:

1. You will need to write or add this snippet of code in every single page that want access to the current logged user.
2. You are force to access to your data using the {{#model}}...{{/model}} expression.

  User user = ...;
  Object model = ...;
  Context context = Context
    .newBuilder(model)
    .combine("user", user)
    .build();

  template.apply(context);

We definitely fix the second drawback described before.

The first one depends on your application architecture. For example, if you have ahookmethod where you can globally merge the model with a view (and of course you can access to the current logged user).

Value resolvers

AValueResolveris responsible of extracting a single value from a source object. By default, only mapsandJavaBeanobjects are supported.

The JavaBeanValueResolver

Let you access to JavaBean properties. It is registered by default.

  Context context = Context
    .newBuilder(model)
    .resolver(JavaBeanValueResolver.INSTANCE)
    .build();

  template.apply(context);

The MapValueResolver

Let you access tomapvalues. It is registered by default.

  Context context = Context
    .newBuilder(model)
    .resolver(MapValueResolver.INSTANCE)
    .build();

  template.apply(context);

The FieldValueResolver

Let you access to instance fields (private or public).

  Context context = Context
    .newBuilder(model)
    .resolver(FieldValueResolver.INSTANCE)
    .build();

  template.apply(context);

The MethodValueResolver

Let you access to public methods.

  Context context = Context
    .newBuilder(model)
    .resolver(MethodValueResolver.INSTANCE)
    .build();

  template.apply(context);

Using multiples value resolvers

You can combine any number of value resolvers according to your application data.

  Context context = Context
    .newBuilder(model)
    .resolver(
        MapValueResolver.INSTANCE,
        JavaBeanValueResolver.INSTANCE
    )
    .build();

  template.apply(context);

  Context context = Context
    .newBuilder(model)
    .resolver(
        MapValueResolver.INSTANCE,
        JavaBeanValueResolver.INSTANCE,
        MethodValueResolver.INSTANCE
    )
    .build();

  template.apply(context);

  Context context = Context
    .newBuilder(model)
    .resolver(
        MapValueResolver.INSTANCE,
        JavaBeanValueResolver.INSTANCE,
        FieldValueResolver.INSTANCE
    )
    .build();

  template.apply(context);

Conclusion

TheContextclass does all the hard work: walk through the stack, resolve expressions, etc. This component isn't puggable and can't be set.

Where aValueResolverextracts a single value from a source. You can add, remove or even create your own value resolver. It plays an important role if your application deal with a custom data source.

For example, if your service layer returns the widely usedJsonNodefrom Jackson a custom ValueResolver can access to that object and extract values from it.

Want to contribute?

• Fork the project on Github.
• Create an issue or fix one from the issues list.
• Share your ideas or ask questions on mailing list - don't hesitate to write a reply - that helps us improve javadocs/FAQ.
• If you miss a particular feature - browse or ask on the mailing list - don't hesitate to write a reply, show us a sample code and describe the problem.
• Write a blog post about how you use or extend Handlebars.java.
• Please suggest changes to javadoc/exception messages when you find something unclear.
• If you have problems with documentation, find it non intuitive or hard to follow - let us know about it, we'll try to make it better according to your suggestions. Any constructive critique is greatly appreciated. Don't forget that this is an open source project developed and documented in spare time.

Thank you, for reading the Handlebars.java blog.