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A successful compile is only one part of a JDK upgrade: the application can still fail at startup or behave differently at runtime. Test on the target JDK first, check dependencies and tools, scan for unsupported or removed APIs, then compile and retest. This step-by-step workflow helps isolate the cause instead of treating every failure as a compiler problem.
1. Run the existing application on the target JDK
Before changing source code or recompiling, run the current application and its existing tests using the JDK you plan to adopt. This can reveal runtime or library incompatibilities separately from issues introduced by recompilation. Oracle recommends treating migration as an iterative process and checking behavior even when the program starts successfully (Oracle’s JDK 26 migration guide).
Record the failures and changes you observe:
- Startup errors, exceptions, and test failures.
- Warnings about obsolete VM options or other runtime configuration.
- Differences in outputs, files, network responses, or other externally visible behavior.
Keep this baseline so you can tell whether each change fixes the original problem or creates a new one.
2. Check libraries, build tools, and IDE support
Review the third-party libraries and development tools used to build and run the application. Oracle’s migration guidance specifically calls out build tools such as Maven and Gradle, and IDEs such as NetBeans, Eclipse, and IntelliJ. Check each project’s release information for support of your chosen JDK; the fact that a tool is in one of these categories does not establish that a particular version supports your target release (Oracle’s JDK 26 migration guide).
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- Update dependencies to versions intended to work with the target JDK.
- Verify that the compiler, build system, test runner, and IDE can use that JDK.
- Change one set of dependencies or tools at a time where practical, then rerun the failing tests.
3. Compile for the Java platform level you intend to support
Use --release when you need to compile against a particular Java platform release. It constrains compilation to that release’s language and platform API surface. For example, compiling with a newer JDK does not by itself ensure the resulting program only uses APIs available on an older runtime; select the intended release explicitly. Oracle recommends --release as part of migration work (Oracle’s JDK 26 next steps).
Compilation can expose source and API problems, but it does not prove the application will run correctly or retain the same behavior. Run tests on the actual target runtime as a separate step.
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4. Find dependencies on internal JDK APIs
Use jdeps with the -jdkinternals option to identify statically visible references to internal JDK APIs in your application and relevant libraries. Replace those references with supported APIs where possible. Oracle’s example replaces sun.misc.BASE64Encoder with java.util.Base64 (Oracle’s JDK 26 migration guide).
A scan cannot find every use: code that accesses internal APIs through reflection may evade static analysis. Treat the output as a useful lead, not proof that no internal API dependencies remain. Also investigate runtime warnings, library behavior, and test failures.
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5. Check deprecated and removed APIs for your target release
Use jdeprscan to investigate deprecated APIs and APIs marked for removal, specifying the release you are targeting. For APIs already removed, consult that release’s migration guide and removed-API inventory. These checks are release-specific: an API available in one JDK may be absent in another.
For example, Oracle’s JDK 25 removed-API documentation gives the release-specific command jdeprscan --release 25 -l --for-removal. Use the corresponding release value and documentation for your target rather than copying this command unchanged for a different JDK (Oracle’s JDK 25 removed APIs; Oracle’s JDK 26 next steps).
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6. Test behavior changes across Java releases
Java upgrades can introduce binary, source, and behavioral incompatibilities. A program may compile and launch yet produce different results because a default or runtime behavior changed (Oracle’s JDK 24 guide to migrating from JDK 8).
Check text encoding when crossing JDK 18
JDK 18 changed the default charset used by Java SE APIs to UTF-8 on all operating systems. JDK 17 and earlier could use an environment-dependent default. If your application reads or writes text without choosing an encoding explicitly, test those paths when upgrading across this boundary. Oracle documents this change in its JDK 21 migration guidance (Oracle’s JDK 21 migration guide).
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- Exercise text imports, exports, and file processing with the encodings your application encounters.
- Check outputs and integrations, not only whether the code completes without an exception.
- Where the required encoding is known, specify it explicitly rather than relying on a platform-dependent default.
7. Repeat the checks after each repair
After updating dependencies, replacing unsupported API calls, adjusting code, or changing build configuration, repeat the relevant checks on the target JDK. A practical sequence is:
- Run the application and tests on the target runtime.
- Update incompatible libraries and toolchain components.
- Compile with
--releasefor the intended platform level. - Run
jdeps -jdkinternalsand investigate its findings. - Run
jdeprscanfor the target release and review that release’s migration guide for removed APIs. - Retest runtime behavior, including paths affected by release-specific changes.
No single scan or successful build establishes complete compatibility. The reliable result comes from combining static checks with tests and observed application behavior on the JDK you intend to use.
