View-Based Projection

View-based projection lets you define named views — property subsets — on a bean and choose which view is active per serialize/parse call. Only the properties belonging to the active view are emitted on serialization; on parsing, out-of-view properties are silently discarded (or rejected, depending on your unknown-property policy).

This is the Juneau equivalent of Jackson's @JsonView / MapperFeature.DEFAULT_VIEW_INCLUSION.

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Declaring view membership on properties

Use the @MarshalledProp(view) member to tag a property with one or more string view names:

public class Person {

    // Untagged — visible in every view (default policy).
    public String id;

    // Visible only when the "summary" view is active.
    @MarshalledProp(view = "summary")
    public String name;

    // Visible only when the "detail" view is active.
    @MarshalledProp(view = "detail")
    public String description;

    // Visible in EITHER "summary" OR "detail" (union semantics).
    @MarshalledProp(view = {"summary", "detail"})
    public String email;
}

View names are plain strings. No marker-class hierarchy is required.

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Activating a view on a serializer or parser

Call activeView(String) on any serializer or parser builder to set a context-level default:

// Serialize with the "summary" view.
var s = Json5Serializer.DEFAULT.copy().activeView("summary").build();
s.write(new Person());
// → {id:'1',name:'Alice',email:'alice@example.com'}

To override the view for a single call without rebuilding the serializer, use activeView(String) on a session:

var session = serializer.createSession().activeView("detail").build();
session.write(new Person());
// → {description:'A person',email:'alice@example.com',id:'1'}

Passing null clears the active view for that session, making all view-tagged properties visible again.

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Default-view policy: untagged properties

When an active view is set, untagged properties are included by default — this matches Jackson's MapperFeature.DEFAULT_VIEW_INCLUSION default and the principle of least surprise: only explicitly-tagged properties become view-restricted.

To flip the policy so that only tagged properties are visible, call disableDefaultViewInclusion():

// Strict mode: only tagged properties appear.
var s = Json5Serializer.DEFAULT.copy()
    .activeView("summary")
    .disableDefaultViewInclusion()
    .build();
s.write(new Person());
// → {email:'alice@example.com',name:'Alice'}   (id omitted — it was untagged)

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View semantics

• Flat string names. No class-hierarchy inheritance; a view named "summary" has no relationship to any other view.
• Union membership. A property tagged {A, B} is visible whenever the active view is A or B. There is no intersection requirement.
• No active view = all properties visible. If no activeView() is set (or it is set to null), the feature is off and every property is serialized as normal.

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View membership on unmodifiable classes (*Config / @ContextApply)

If you cannot annotate a class directly (third-party, generated, or sealed), apply view membership via the @MarshalledPropApply *Config mechanism:

// Config annotation — does not require modifying ThirdPartyBean.
@MarshalledPropApply(on = "ThirdPartyBean.name",        value = @MarshalledProp(view = "summary"))
@MarshalledPropApply(on = "ThirdPartyBean.description", value = @MarshalledProp(view = "detail"))
public static class ThirdPartyBeanViewConfig {}

// Apply the config when building the serializer.
var s = Json5Serializer.DEFAULT.copy()
    .applyAnnotations(ThirdPartyBeanViewConfig.class)
    .activeView("summary")
    .build();

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Parse-side behavior

When parsing, properties that are not in the active view are treated as unknown properties. By default Juneau silently discards unknown properties, so out-of-view input is quietly ignored:

var p = Json5Parser.DEFAULT.copy().activeView("summary").build();
// description is not in the "summary" view → ignored (default behavior).
Person person = p.read("{id:'1',name:'Alice',description:'ignored'}", Person.class);

To make out-of-view input throw an exception, disable the unknown-property tolerance with ignoreUnknownBeanProperties(false):

var p = Json5Parser.DEFAULT.copy()
    .activeView("summary")
    .ignoreUnknownBeanProperties(false)  // strict: out-of-view input throws
    .build();
p.read("{id:'1',description:'oops'}", Person.class);  // throws ParseException

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Precedence with other property controls

When multiple property-control mechanisms interact, the precedence is:

1. @MarshalledIgnore / @BeanIgnore — hard exclude (wins over everything).
2. readOnly / writeOnly — direction filter.
3. Active view — membership filter.
4. beanProperties / beanPropertiesExcludes — further narrowing.
5. Swaps / value-level inclusion knobs — applied to surviving properties.

Views and beanProperties* compose by intersection: a property must pass both filters to appear.

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Cross-format invariance

View selection lives in MarshallingContext / BeanPropertyMeta — the bean model layer — not in any format-specific serializer. The same activeView() call produces consistent projection across JSON, XML, HTML, MsgPack, YAML, and every other Juneau format.

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Migration from Jackson @JsonView

Jackson	Juneau
@JsonView(MyView.class) on a property	@MarshalledProp(view = "MyView")
@JsonView({A.class, B.class})	@MarshalledProp(view = {"A", "B"})
objectMapper.writerWithView(MyView.class).writeValue(...)	serializer.createSession().activeView("MyView").build().write(...)
objectMapper.readerWithView(MyView.class).readValue(...)	parser.createSession().activeView("MyView").build().read(...)
MapperFeature.DEFAULT_VIEW_INCLUSION = true (default)	default — untagged properties included
MapperFeature.DEFAULT_VIEW_INCLUSION = false	serializer.copy().activeView(...).disableDefaultViewInclusion().build()
Marker-class view tokens (class SummaryView {})	Not needed — Juneau uses plain strings
View inheritance (class DetailView extends SummaryView)	No equivalent — Juneau views are flat names; list both: view = {"summary", "detail"}