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# StrutsParameter Annotation

`@StrutsParameter` is a security annotation that marks which fields and methods in your Action class can receive values from user requests.

Why it matters: by default (when annotations are required), Struts will only inject request parameters into fields or setter methods that have this annotation. This prevents attackers from setting values on fields you didn’t intend to expose.

## Where authorization applies

As of Struts 7.2.0 the `@StrutsParameter` authorization is enforced across every channel that can populate an action from request data:

- Parameters Interceptor — request parameters (default, governed by `struts.parameters.requireAnnotations`).
- Chaining Interceptor — value-stack copying during action chaining (opt-in via `struts.chaining.requireAnnotations`).
- Cookie Interceptor — cookie values.
- JSON and REST plugins — per-property authorization performed during deserialization, so unauthorized fields are never set.

## ModelDriven actions

When an action implements `ModelDriven` and the Model Driven Interceptor has pushed the model onto the value stack, the model — not the action — is the authorization target, and the model’s members are exempt from the annotation requirement. The whole model is bindable, including its nested properties, whether or not any of its fields or accessors carry `@StrutsParameter`.

This follows from what the interface declares: returning an object from `getModel()` designates that object as the request surface. The model has been the authorization target since Struts 7.0.0.

The exemption holds on every input channel listed above — request parameters, JSON bodies and REST bodies alike — because they all resolve the authorization target the same way. It is not a JSON- or REST-specific behavior.

The exemption is narrowly scoped. It applies only when the action itself implements `ModelDriven` and the object being populated is its model rather than the action. A root object configured elsewhere — for example the JSON interceptor’s `root` expression on an action that does not implement `ModelDriven` — is not exempt, and each member it binds still requires `@StrutsParameter`.

The exemption lifts the annotation requirement only. The other parameter-name checks — accepted and excluded name patterns, and `ParameterNameAware` — still apply to a model’s parameters as they do to an action’s.

Because the entire model is bindable, a `ModelDriven` model should be a request DTO carrying only the fields the action intends to accept from a request, never a domain or persistence entity. If you need member-level control over what is bindable, use action properties annotated with `@StrutsParameter` rather than `ModelDriven`.

## Usage

The placement of the `@StrutsParameter` annotation is crucial and depends on how you want to populate your action properties.

- On a public setter method: Place the annotation on a setter method when you want to populate the property with a value from the request. This applies to:
  - Simple types (String, int, boolean, etc.).
  - Checkboxes (single or multiple values).
  - Collections and Maps, when you are populating the whole collection/map from the request.

- On a public getter method: Place the annotation on a getter method when you want to allow populating the properties of the object (or objects) returned by the getter. The `depth` parameter controls how deep into that object graph population is allowed — see Understanding the `depth` parameter below. This is typically used for complex objects or collections of complex objects.

- On a public field: For simple types, you can place the annotation directly on the public field as a shorthand for a setter annotation.

## Understanding the `depth` parameter

When you annotate a getter, `depth` limits how far Struts may traverse the object graph reachable from that getter while applying request parameters. Each navigation step counts as one level — following a property or indexing into a collection or map.

To find the value you need, count the segments after the annotated property in the request expression:

Request expression | Annotated getter | Steps beyond the getter | Required `depth`
--- | --- | --- | ---
`user.name` | `getUser()` | `.name` | `1`
`user.address.city` | `getUser()` | `.address` → `.city` | `2`
`users[0].name` | `getUsers()` | `[0]` → `.name` | `2`

The key point for collections and maps: indexing into the collection is itself a level. Reaching a property of a collection element therefore always costs one more level than reaching the same property on a plain object. This holds even when the element type is a flat POJO with only simple fields — populating `contents[0].title` still needs `depth = 2` (one level to reach the element, one more to reach its property), not `depth = 1`.

In the annotation’s own terms, `depth` is the number of periods or brackets that may appear in the parameter name. The default is `depth = 0`, which permits only setters and fields directly on the action class. Reaching a property of a returned object needs `depth = 1` or more; reaching a property of an object held in a collection or map needs `depth = 2` or more.

## Examples

### Simple field

Annotating the field:

    public class MyAction {
        @StrutsParameter
        public String username;  // ✅ Can receive request parameter
    }

Annotating the setter:

    public class MyAction {
        private String username;
        @StrutsParameter
        public void setUsername(String username) {
            this.username = username;
        }
    }

### Checkbox

For a single checkbox, the annotation must be on the setter.

    public class MyAction {
        private boolean myCheckbox;

        @StrutsParameter
        public void setMyCheckbox(boolean myCheckbox) {
            this.myCheckbox = myCheckbox;
        }
        // ... getter
    }

### Collections

#### Populating a collection of simple types

When populating a collection of simple types (e.g., from a checkbox list), annotate the setter.

    public class MyAction {
        private List<String> mySelection;

        @StrutsParameter
        public void setMySelection(List<String> mySelection) {
            this.mySelection = mySelection;
        }
        // ... getter
    }

This covers the case where the whole collection is assigned at once (name `mySelection`, `depth = 0`), as a checkbox list submits it.

When the collection is instead populated element by element through indexed names — `mySelection[0]`, `mySelection[1]` — the annotation must be on the getter with `depth = 1`, because each element path contains one bracket. This is how JSON and REST payloads bind a collection of simple types: a body such as `{"mySelection":["A","B"]}` populates `mySelection[0]` and `mySelection[1]`, so the getter must be annotated for the elements to be accepted.

    public class MyAction {
        private List<String> mySelection;

        @StrutsParameter(depth = 1)
        public List<String> getMySelection() {
            return mySelection;
        }
        // ... setter
    }

When populating properties of objects that are already in a collection, annotate the getter. Because reaching an element’s property requires indexing into the collection and then following the property, this needs `depth = 2` (see Understanding the `depth` parameter).

    public class MyAction {
        private List<User> users; // assume this is initialized in the constructor or elsewhere

        @StrutsParameter(depth = 2)
        public List<User> getUsers() {
            return users;
        }
        // ...
    }

This allows requests like `users[0].name=John`. Note that `depth = 2` is required even when `User` is a flat object with only simple properties — the extra level pays for indexing into the collection, not for nesting within the element.

The same rule applies to JSON and REST payloads: a body such as `{"users":[{"name":"John"}]}` populates `users[0].name`, so the `getUsers()` getter must be annotated with `depth = 2` for the nested value to be accepted. Annotating only the setter is not enough — the JSON/REST authorization checks the getter when descending into the collection’s elements.

### Complex object

#### Populating the object itself

To populate the whole object from the request (e.g., using a custom type converter), annotate the setter.

    public class MyAction {
        private User user;

        @StrutsParameter
        public void setUser(User user) {
            this.user = user;
        }
        // ... getter
    }

#### Populating properties of a complex object

To populate the properties of a complex object, annotate the getter.

    public class MyAction {
        private User user = new User();

        @StrutsParameter(depth = 1)
        public User getUser() {
            return user;
        }
    }

This allows requests like `user.name=John`.

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