Is SPI safe to use in multi-threaded PL/Java?

Hello,

Is PL/Java safe to use in terms of its threading design? I'm going to ask
the PL/Java community about this too, but I'd ask for opinions here because
I believe people in this community have seasoned knowledge of OS and SPI.

To put the question in other words, is it safe to load a multi-threaded PL
library in the single-threaded backend process, if the PL only calls SPI in
the main thread?

PL/Java (pljava.so) is linked with the JNI (Java Native Interface) library,
libjvm.so, in JRE. libjvm.so is linked with libpthread.so, because Java VM
is multi-threaded. SO, "ldd pljava.so" shows libjvm.so and libpthread.so.
pljava.so doesn't seem to be built for multh-threading --- none
of -mt, -D_REENTRANT or -D_POSIX_C_SOURCE is specified when building it.

When the application calls Java stored function, pljava.so calls a function
in libjvm.so to create a JVM in the backend process, then invokes the
user-defined Java method in the main thread. The user-defined Java method
calls JDBC methods to access database. The JDBC method calls are translated
to backend SPI function calls through JNI.

The main thread can create Java threads using Java Thread API, and those
threads can call JDBC methods. However, PL/Java intercepts JDBC method
calls and serializes SPI calls. So, only one thread calls SPI functions at
a time. I'm wondering if this is the reason why PL/Java is safe for use.

What I'm concerned about is whether multi-threaded code (Java VM) can run
safely in a single-threaded code (postgres). I don't know what can be a
particular problem with PL/Java, but in general, the mixture of
single-threaded code and multi-threaded one seems to cause trouble around
handling errno, memory and file handles/pointers.

FYI, JNI specification says that the code called from Java VM should be
built for multi-threading as follows. But postgres is not.

http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/design.html#wp9502

[Excerpt]
Compiling, Loading and Linking Native Methods
Since the Java VM is multithreaded, native libraries should also be compiled
and linked with multithread aware native compilers. For example, the -mt
flag should be used for C++ code compiled with the Sun Studio compiler. For
code complied with the GNU gcc compiler, the flags -D_REENTRANT
or -D_POSIX_C_SOURCE should be used. For more information please refer to
the native compiler documentation.

Regards
MauMau