Java 25 includes Stable Values, a preview API for holding a value that can be initialized later but set only once. Oracle says the JVM can treat an assigned stable value like a constant for optimization, while allowing more flexible initialization timing than a final field. The API is not final: using it requires preview features to be enabled, and its design may change in a later Java release.
What are stable values in Java 25?
StableValue<T> is a holder for a value that can start unset and then be assigned at most once. After assignment, the holder cannot be used to replace its contents. Oracle describes the API as shallowly immutable: the holder’s reference or value cannot be replaced through the holder, but this does not make an object stored in it deeply immutable.
The intended use is a value that is costly or inconvenient to initialize immediately, yet should not change once it has been established. Oracle’s stated rationale is that the JVM may treat stable values as constants and apply the same performance optimizations as for a final field. This is a design claim, not a quantified benchmark result; performance depends on the program and workload.
See Oracle’s Java SE 25 StableValue API documentation for the exact method contracts.
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How do I use StableValue?
Create a holder with StableValue.of(). It begins unset. The API offers methods for attempting to set it, requiring a successful set, or computing and setting a value only if one is absent:
trySet(value)attempts to set the value, without throwing if it cannot set it.setOrThrow(value)sets the value or throws if the holder has already been set.orElseSet(supplier)obtains and sets a value if the holder is unset; if already set, it uses the existing value.
For retrieval, the API includes orElseThrow() when a value must be present, orElse(fallback) when a fallback is suitable, and orElseSet(supplier) when retrieval should initialize an unset holder. Consult the API documentation for precise behavior, exceptions, and concurrency semantics before choosing a method.
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StableValue vs. a final field
| Consideration | final field |
StableValue in JDK 25 |
|---|---|---|
| Initialization timing | Must be assigned as part of field initialization or construction. | Can be created unset and assigned later. |
| Changing the held value | Cannot be reassigned after initialization. | Can be set at most once through the holder; it cannot later replace the held value. |
| JVM optimization intent | Final fields are the stated comparison for constant-like optimization. | Oracle says the JVM can treat stable values as constants and enable the same performance optimizations; no numeric improvement is established. |
| Maturity and deployment | Established Java language feature; no preview flag is needed for the field itself. | Preview API in Java 25; preview features must be enabled, and the API may change, be removed, or become permanent in a later release. |
The choice is primarily about initialization design, not a guaranteed speedup. A final field remains straightforward when construction can supply the value. A stable holder is relevant when initialization needs to happen later; benchmark the actual workload before drawing performance conclusions.
Is JEP 502 final or preview?
JEP 502 Stable Values is a preview feature in Java 25, not a finalized platform API. Programs that use it must enable preview features. Oracle’s Java SE 25 preview list describes the preview status; preview features may be removed in a future release or upgraded to permanent platform features.
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That status matters for compatibility. Teams considering use should account for preview-enabled builds and runtime configuration, then retest and review migration requirements when moving to a later Java release. Do not treat code that compiles against JDK 25 as proof that the API will remain unchanged.
What else is notable in JDK 25?
Oracle announced Java 25 on September 16, 2025. The release also highlighted changes across the language, libraries, runtime, and security. Their maturity differs, so not every item below has the same status as Stable Values.
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- Language: module import declarations; compact source files and instance main methods; flexible constructor bodies; and primitive types in patterns,
instanceof, andswitch(preview). - Libraries: scoped values and key derivation functions; the Vector API is an incubator feature, while PEM encodings are preview.
- Runtime and performance: compact object headers and ahead-of-time improvements.
Oracle’s Java 25 announcement and JDK 25 migration notes provide release context and migration details.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which JDK 25 update should I use?
Java 25 is a release line, not a single frozen build. Oracle’s consolidated JDK 25 release notes list JDK 25.0.4.1 GA, version 25.0.4.1+1, released August 18, 2026, and recommend keeping JDK installations current with Critical Patch Updates. Check Oracle’s release notes for a newer update before installing or upgrading; the listed update and date can change.
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