lavc/ffjni: replace ff_jni_{attach,detach} with ff_jni_get_env

If a JNI environment is not already attached to the thread where the
MediaCodec calls are made the current implementation will attach /
detach an environment for each MediaCodec call wasting some CPU time.

ff_jni_get_env replaces ff_jni_{attach,detach} by permanently attaching
an environment (if it is not already the case) to the current thread.
The environment will be automatically detached at the thread destruction
using a pthread_key callback.

Saves around 5% of CPU time (out of 20%) while decoding a stream with
MediaCodec.

libavcodec/ffjni.c

@@ -31,25 +31,42 @@
 #include "jni.h"
 #include "ffjni.h"
 
-static JavaVM *java_vm = NULL;
+static JavaVM *java_vm;
+static pthread_key_t current_env;
+static pthread_once_t once = PTHREAD_ONCE_INIT;
 static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
 
-JNIEnv *ff_jni_attach_env(int *attached, void *log_ctx)
+static void jni_detach_env(void *data)
+{
+    if (java_vm) {
+        (*java_vm)->DetachCurrentThread(java_vm);
+    }
+}
+
+static void jni_create_pthread_key(void)
+{
+    pthread_key_create(&current_env, jni_detach_env);
+}
+
+JNIEnv *ff_jni_get_env(void *log_ctx)
 {
     int ret = 0;
     JNIEnv *env = NULL;
 
-    *attached = 0;
-
     pthread_mutex_lock(&lock);
     if (java_vm == NULL) {
         java_vm = av_jni_get_java_vm(log_ctx);
     }
-    pthread_mutex_unlock(&lock);
 
     if (!java_vm) {
         av_log(log_ctx, AV_LOG_ERROR, "No Java virtual machine has been registered\n");
-        return NULL;
+        goto done;
+    }
+
+    pthread_once(&once, jni_create_pthread_key);
+
+    if ((env = pthread_getspecific(current_env)) != NULL) {
+        goto done;
     }
 
     ret = (*java_vm)->GetEnv(java_vm, (void **)&env, JNI_VERSION_1_6);
@@ -59,7 +76,7 @@ JNIEnv *ff_jni_attach_env(int *attached, void *log_ctx)
             av_log(log_ctx, AV_LOG_ERROR, "Failed to attach the JNI environment to the current thread\n");
             env = NULL;
         } else {
-            *attached = 1;
+            pthread_setspecific(current_env, env);
         }
         break;
     case JNI_OK:
@@ -72,19 +89,11 @@ JNIEnv *ff_jni_attach_env(int *attached, void *log_ctx)
         break;
     }
 
+done:
+    pthread_mutex_unlock(&lock);
     return env;
 }
 
-int ff_jni_detach_env(void *log_ctx)
-{
-    if (java_vm == NULL) {
-        av_log(log_ctx, AV_LOG_ERROR, "No Java virtual machine has been registered\n");
-        return AVERROR(EINVAL);
-    }
-
-    return (*java_vm)->DetachCurrentThread(java_vm);
-}
-
 char *ff_jni_jstring_to_utf_chars(JNIEnv *env, jstring string, void *log_ctx)
 {
     char *ret = NULL;

libavcodec/ffjni.h

@@ -26,7 +26,10 @@
 #include <jni.h>
 
 /*
- * Attach a JNI environment to the current thread.
+ * Attach permanently a JNI environment to the current thread and retrieve it.
+ *
+ * If successfully attached, the JNI environment will automatically be detached
+ * at thread destruction.
  *
  * @param attached pointer to an integer that will be set to 1 if the
  * environment has been attached to the current thread or 0 if it is
@@ -34,15 +37,7 @@
  * @param log_ctx context used for logging, can be NULL
  * @return the JNI environment on success, NULL otherwise
  */
-JNIEnv *ff_jni_attach_env(int *attached, void *log_ctx);
-
-/*
- * Detach the JNI environment from the current thread.
- *
- * @param log_ctx context used for logging, can be NULL
- * @return 0 on success, < 0 otherwise
- */
-int ff_jni_detach_env(void *log_ctx);
+JNIEnv *ff_jni_get_env(void *log_ctx);
 
 /*
  * Convert a jstring to its utf characters equivalent.

libavcodec/mediacodec.c

@@ -43,9 +43,8 @@ int av_mediacodec_default_init(AVCodecContext *avctx, AVMediaCodecContext *ctx,
 {
     int ret = 0;
     JNIEnv *env = NULL;
-    int attached = 0;
 
-    env = ff_jni_attach_env(&attached, avctx);
+    env = ff_jni_get_env(avctx);
     if (!env) {
         return AVERROR_EXTERNAL;
     }
@@ -58,17 +57,12 @@ int av_mediacodec_default_init(AVCodecContext *avctx, AVMediaCodecContext *ctx,
         ret = AVERROR_EXTERNAL;
     }
 
-    if (attached) {
-        ff_jni_detach_env(avctx);
-    }
-
     return ret;
 }
 
 void av_mediacodec_default_free(AVCodecContext *avctx)
 {
     JNIEnv *env = NULL;
-    int attached = 0;
 
     AVMediaCodecContext *ctx = avctx->hwaccel_context;
 
@@ -76,7 +70,7 @@ void av_mediacodec_default_free(AVCodecContext *avctx)
         return;
     }
 
-    env = ff_jni_attach_env(&attached, avctx);
+    env = ff_jni_get_env(avctx);
     if (!env) {
         return;
     }
@@ -86,10 +80,6 @@ void av_mediacodec_default_free(AVCodecContext *avctx)
         ctx->surface = NULL;
     }
 
-    if (attached) {
-        ff_jni_detach_env(avctx);
-    }
-
     av_freep(&avctx->hwaccel_context);
 }
 

libavcodec/mediacodec_surface.c

@@ -27,40 +27,30 @@
 
 void *ff_mediacodec_surface_ref(void *surface, void *log_ctx)
 {
-    int attached = 0;
     JNIEnv *env = NULL;
 
     void *reference = NULL;
 
-    env = ff_jni_attach_env(&attached, log_ctx);
+    env = ff_jni_get_env(log_ctx);
     if (!env) {
         return NULL;
     }
 
     reference = (*env)->NewGlobalRef(env, surface);
 
-    if (attached) {
-        ff_jni_detach_env(log_ctx);
-    }
-
     return reference;
 }
 
 int ff_mediacodec_surface_unref(void *surface, void *log_ctx)
 {
-    int attached = 0;
     JNIEnv *env = NULL;
 
-    env = ff_jni_attach_env(&attached, log_ctx);
+    env = ff_jni_get_env(log_ctx);
     if (!env) {
         return AVERROR_EXTERNAL;
     }
 
     (*env)->DeleteGlobalRef(env, surface);
 
-    if (attached) {
-        ff_jni_detach_env(log_ctx);
-    }
-
     return 0;
 }