Merge branch 'java-director-leak'

* java-director-leak:
  Cosmetic tidyup in SWIG_JAVA_DETACH_ON_THREAD_END code
  Java: Move auxiliary methods into JObjectWrapper.
  Java: Option to detach from the JVM in the thread destructor.

Conflicts:
	CHANGES.current

Author: wsfulton

CHANGES.current

@@ -7,6 +7,16 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-05-28: jkuebart
+            [Java] On some versions of Android, specifically Android 6,
+            detaching the current thread from the JVM after every invocation
+            causes a memory leak.
+
+            Offer SWIG_JAVA_DETACH_ON_THREAD_END to configure a behaviour
+            where the JVM is only detached in the thread destructor.
+
+            See https://developer.android.com/training/articles/perf-jni#threads.
+
 2022-05-27: xypron
             [Python] #2277 Define PY_SSIZE_T_CLEAN macro before #include "Python.h" as
             recommended in Python 3.7 and later.

Lib/java/director.swg

@@ -51,6 +51,10 @@ SWIGINTERN int Swig::GetThreadName(char *name, size_t len) {
 
 #endif
 
+#if defined(SWIG_JAVA_DETACH_ON_THREAD_END)
+#include <pthread.h>
+#endif
+
 namespace Swig {
 
   /* Java object wrapper */
@@ -133,13 +137,30 @@ namespace Swig {
       }
     }
 
+#if defined(SWIG_JAVA_DETACH_ON_THREAD_END)
+    static void detach(void *jvm) {
+      static_cast<JavaVM *>(jvm)->DetachCurrentThread();
+    }
+
+    static void make_detach_key() {
+      pthread_key_create(&detach_key_, detach);
+    }
+
+    /* thread-local key to register a destructor */
+    static pthread_key_t detach_key_;
+#endif
+
   private:
     /* pointer to Java object */
     jobject jthis_;
     /* Local or global reference flag */
     bool weak_global_;
   };
 
+#if defined(SWIG_JAVA_DETACH_ON_THREAD_END)
+  pthread_key_t JObjectWrapper::detach_key_;
+#endif
+
   /* Local JNI reference deleter */
   class LocalRefGuard {
     JNIEnv *jenv_;
@@ -201,9 +222,19 @@ namespace Swig {
 #else
         director_->swig_jvm_->AttachCurrentThread(jenv, &args);
 #endif
+
+#if defined(SWIG_JAVA_DETACH_ON_THREAD_END)
+        // At least on Android 6, detaching after every call causes a memory leak.
+        // Instead, register a thread desructor and detach only when the thread ends.
+        // See https://developer.android.com/training/articles/perf-jni#threads
+        static pthread_once_t once = PTHREAD_ONCE_INIT;
+
+        pthread_once(&once, JObjectWrapper::make_detach_key);
+        pthread_setspecific(JObjectWrapper::detach_key_, director->swig_jvm_);
+#endif
       }
       ~JNIEnvWrapper() {
-#if !defined(SWIG_JAVA_NO_DETACH_CURRENT_THREAD)
+#if !defined(SWIG_JAVA_DETACH_ON_THREAD_END) && !defined(SWIG_JAVA_NO_DETACH_CURRENT_THREAD)
         // Some JVMs, eg jdk-1.4.2 and lower on Solaris have a bug and crash with the DetachCurrentThread call.
         // However, without this call, the JVM hangs on exit when the thread was not created by the JVM and creates a memory leak.
         if (env_status == JNI_EDETACHED)