语言的设计是石头铸造的（译注：原文为“cast in stone”，意为“最终定论，板上钉钉”，此处双关）， 但这块石头相当柔软， 经过一番努力，我们可以后期重塑它。 Kotlin 设计团队
由于这是理解 Kotlin 如何向前发展的关键，我们来展开来看这些原则。
舒适的更新。如果没有适度谨慎地进行不兼容的变更（例如从语言中删除内容）可能会导致从一个版本到下一个版本的痛苦迁移过程。我们会始终提前公布这类变更，将相应内容标记为已弃用并 在变更发生之前 提供自动化的迁移工具。当语言发生变更之时，我们希望世界上绝大多数代码都已经更新，这样迁移到新版本就没有问题了。
反馈循环。 通过弃用周期需要付出很大的努力，因此我们希望最大限度地减少将来不兼容变更的数量。除了使用我们的最佳判断之外，我们相信在现实生活中试用是验证设计的最佳方法。在最终定论之前，我们希望已经实战测试过。这就是为什么我们利用每个机会在语言的生产版本中提供我们早期版设计，只是带有_实验性_ 状态。实验性特性并不稳定，可以随时更改，选择使用它们的用户明确表示已准备好了应对未来的迁移问题。这些用户提供了宝贵的反馈，而我们收集这些反馈来迭代设计并使其坚如磐石。
如果从一个版本更新到另一个版本时，一些以前工作的代码不再工作，那么它是语言中的 不兼容的变更 （有时称为“破坏性变更”）。在一些场景中“不再工作”的确切含义可能会有争议，但是它肯定包含以下内容：
The less obvious cases that belong to the "grey area" include handling corner cases differently, throwing an exception of a different type than before, changing behavior observable only through reflection, changes in undocumented/undefined behavior, renaming binary artifacts, etc. Sometimes such changes are very important and affect migration experience dramatically, sometimes they are insignificant.
- Adding new warnings.
- Enabling new language constructs or relaxing limitations for existing ones.
- Changing private/internal APIs and other implementation details.
The principles of Keeping the Language Modern and Comfortable Updates suggest that incompatible changes are sometimes necessary, but they should be introduced carefully. Our goal is to make the users aware of upcoming changes well in advance to let them migrate their code comfortably.
Ideally, every incompatible change should be announced through a compile-time warning reported in the problematic code (usually referred to as a deprecation warning) and accompanied with automated migration aids. So, the ideal migration workflow goes as follows:
- Update to version A (where the change is announced)
- See warnings about the upcoming change
- Migrate the code with the help of the tooling
- Update to version B (where the change happens)
- See no issues at all
In practice some changes can't be accurately detected at compile time, so no warnings can be reported, but at least the users will be notified through Release notes of version A that a change is coming in version B.
Dealing with compiler bugs
Compilers are complicated software and despite the best effort of their developers they have bugs. The bugs that cause the compiler itself to fail or report spurious errors or generate obviously failing code, though annoying and often embarrassing, are easy to fix, because the fixes do not constitute incompatible changes. Other bugs may cause the compiler to generate incorrect code that does not fail: e.g. by missing some errors in the source or simply generating wrong instructions. Fixes of such bugs are technically incompatible changes (some code used to compile fine, but now it won't any more), but we are inclined to fixing them as soon as possible to prevent the bad code patterns from spreading across user code. In our opinion, this serves the principle of Comfortable Updates, because fewer users have a chance of encountering the issue. Of course, this applies only to bugs that are found soon after appearing in a released version.
All changes to the Kotlin Programming Language are overseen by the Lead Language Designer (currently Andrey Breslav). The Lead Designer has the final say in all matters related to language evolution. Additionally, incompatible changes to fully stable components have to be approved to by the Language Committee designated under the Kotlin Foundation (currently comprised of Jeffrey van Gogh, William R. Cook and Andrey Breslav).
The Language Committee makes final decisions on what incompatible changes will be made and what exact measures should be taken to make user updates comfortable. In doing so, it relies on a set of guidelines available here.
Feature Releases and Incremental Releases
Stable releases with versions 1.2, 1.3, etc. are usually considered to be feature releases bringing major changes in the language. Normally, we publish incremental releases, numbered 1.2.20, 1.2.30, etc, in between feature releases.
Incremental releases bring updates in the tooling (often including features), performance improvements and bug fixes. We try to keep such versions compatible with each other, so changes to the compiler are mostly optimizations and warning additions/removals. Experimental features may, of course, be added, removed or changed at any time.
Feature releases often add new features and may remove or change previously deprecated ones. Feature graduation from experimental to stable also happens in feature releases.
Before releasing stable versions, we usually publish a number of preview builds dubbed EAP (for "Early Access Preview") that let us iterate faster and gather feedback from the community. EAPs of feature releases usually produce binaries that will be later rejected by the stable compiler to make sure that possible bugs in the binary format survive no longer than the preview period. Final Release Candidates normally do not bear this limitation.
According to the Feedback Loop principle described above, we iterate on our designs in the open and release versions of the language where some features have the experimental status and are supposed to change. Experimental features can be added, changed or removed at any point and without warning. We make sure that experimental features can't be used accidentally by an unsuspecting user. Such features usually require some sort of an explicit opt-in either in the code or in the project configuration.
Experimental features usually graduate to the stable status after some iterations.
Status of different components
To check the stability status of different components of Kotlin (Kotlin/JVM, JS, Native, various libraries, etc), please consult this link.
A language is nothing without its ecosystem, so we pay extra attention to enabling smooth library evolution.
Ideally, a new version of a library can be used as a "drop-in replacement" for an older version. This means that upgrading a binary dependency should not break anything, even if the application is not recompiled (this is possible under dynamic linking).
On the one hand, to achieve this, the compiler has to provide certain ABI stability guarantees under the constraints of separate compilation. This is why every change in the language is examined from the point of view of binary compatibility.
On the other hand, a lot depends on the library authors being careful about which changes are safe to make. Thus it's very important that library authors understand how source changes affect compatibility and follow certain best practices to keep both APIs and ABIs of their libraries stable. Here are some assumptions that we make when considering language changes from the library evolution standpoint:
- Library code should always specify return types of public/protected functions and properties explicitly thus never relying on type inference for public API. Subtle changes in type inference may cause return types to change inadvertently, leading to binary compatibility issues.
- Overloaded functions and properties provided by the same library should do essentially the same thing. Changes in type inference may result in more precise static types to be known at call sites causing changes in overload resolution.
Library authors can use the @Deprecated and @Experimental annotations to control the evolution of their API surface. Note that @Deprecated(level=HIDDEN) can be used to preserve binary compatibility even for declarations removed from the API.
Also, by convention, packages named "internal" are not considered public API. All API residing in packages named "experimental" is considered experimental and can change at any moment.
We evolve the Kotlin Standard Library (kotlin-stdlib) for stable platforms according to the principles stated above. Changes to the contracts for its API undergo the same procedures as changes in the language itself.
Command line keys accepted by the compiler are also a kind of public API, and they are subject to the same considerations. Supported flags (those that don't have the "-X" or "-XX" prefix) can be added only in feature releases and should be properly deprecated before removing them. The "-X" and "-XX" flags are experimental and can be added and removed at any time.
As legacy features get removed and bugs fixed, the source language changes, and old code that has not been properly migrated may not compile any more. The normal deprecation cycle allows a comfortable period of time for migration, and even when it's over and the change ships in a stable version, there's still a way to compile unmigrated code.
We provide the -language-version and -api-version flags that make a new version emulate the behaviour of an old one, for compatibility purposes. Normally, at least one previous version is supported. This effectively leaves a time span of two full feature release cycles for migration (which usually amounts to about two years). Using an older kotlin-stdlib or kotlin-reflect with a newer compiler without specifying compatibility flags is not recommended, and the compiler will report a warning when this happens.
Actively maintained code bases can benefit from getting bug fixes ASAP, without waiting for a full deprecation cycle to complete. Currently such project can enable the -progressive flag and get such fixes enabled even in incremental releases.
Evolving the binary format
Unlike sources that can be fixed by hand in the worst case, binaries are a lot harder to migrate, and this makes backwards compatibility very important in the case of binaries. Incompatible changes to binaries can make updates very uncomfortable and thus should be introduced with even more care than those in the source language syntax.
For fully stable versions of the compiler the default binary compatibility protocol is the following:
- All binaries are backwards compatible, i.e. a newer compiler can read older binaries (e.g. 1.3 understands 1.0 through 1.2),
- Older compilers reject binaries that rely on new features (e.g. a 1.0 compiler rejects binaries that use coroutines).
- Preferably (but we can't guarantee it), the binary format is mostly forwards compatible with the next feature release, but not later ones (in the cases when new features are not used, e.g. 1.3 can understand most binaries from 1.4, but not 1.5).
This protocol is designed for comfortable updates as no project can be blocked from updating its dependencies even if it's using a slightly outdated compiler.
Please note that not all target platforms have reached this level of stability (but Kotlin/JVM has).