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Improve docs for rules_jni_init #61

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Mar 17, 2022
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Improve docs for rules_jni_init #61

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12 changes: 5 additions & 7 deletions docs/targets.md
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Expand Up @@ -22,9 +22,6 @@ current target platform.
This library can be added to the `deps` of a `cc_*` target that requires access to
the [Java Invocation API](https://docs.oracle.com/en/java/javase/17/docs/specs/jni/invocation.html).

**Note:** Every `cc_binary` that transitively depends on this library should also directly depend on it and call
`rules_jni_init` from its `main` function, providing `argv[0]`. This ensures that

This target serves as a drop-in replacement for the `jvm` shared library contained in JDKs and JREs, which is
called `jvm.dll`, `libjvm.dylib`
or `libjvm.so` depending on the OS. These libraries are usually not available in the standard locations searched by the
Expand All @@ -37,12 +34,13 @@ runtime when its first symbol is accessed. Concretely, it does the following in
finds a runfiles directory or manifest, locate the current Bazel Java runtime in the runfiles and look for the `jvm`
shared library relative to it at well-known locations. If it cannot be found, print a warning and continue.

**Note:** If you want this behavior also for binaries executed via `bazel run` or directly from `bazel-bin`, you have
to:
**Note:** To ensure that the current Bazel Java runtime is also found if the binary is executed via `bazel run` or
directly from `bazel-bin` and to ensure that Java code can find its runfiles, do the following:

1. Depend on `@fmeum_rules_jni//jni:libjvm` directly from your top-level `cc_binary`.
1. Depend on `@fmeum_rules_jni//jni:libjvm` directly from the top-level `cc_binary`.
2. Add `#include <rules_jni.h>`.
3. Call `rules_jni_init(const char* argv0)` from your `main` function, providing `argv[0]` as the argument.
3. Call `rules_jni_init(const char* argv0)` from the `main` function, providing `argv[0]` as the argument. This
call is **not thread-safe** as it modifies the environment.

If you want this lookup to succeed also for binaries executed by other binaries that are themselves run from Bazel,
[set the environment variables required for runfiles discovery](https://github.com/bazelbuild/bazel/blob/e8a066e9e625a136363338d10f03ed14c26dedfa/tools/cpp/runfiles/runfiles_src.h#L58).
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