Android: lazy initialization for method id.

base/android/jni_android.h

Index: base/android/jni_android.h
diff --git a/base/android/jni_android.h b/base/android/jni_android.h
index 492f11ff90bbcafbd70a8a8f659287eddd94d23a..69bd36c0ac74c087ce4f1930ff9f03f501103b07 100644
--- a/base/android/jni_android.h
+++ b/base/android/jni_android.h
@@ -9,7 +9,9 @@
 #include <sys/types.h>
 
 #include "base/android/scoped_java_ref.h"
+#include "base/atomicops.h"
 #include "base/compiler_specific.h"
+#include "base/logging.h"
 
 namespace base {
 namespace android {
@@ -136,6 +138,46 @@ bool ClearException(JNIEnv* env);
 // This function will call CHECK() macro if there's any pending exception.
 void CheckException(JNIEnv* env);
 
+class LazyMethodID {
+ public:
+  enum MethodType {
+    METHODTYPE_STATIC,
+    METHODTYPE_NORMAL,
+  };
+
+  enum ExceptionCheck {
+    EXCEPTIONCHECK_YES,

[Sami, 2012/10/02 15:14:05]

Super-nit: NO=0, YES=1 would seem more logical.

[bulach, 2012/10/02 15:28:30]

:) done..

+    EXCEPTIONCHECK_NO,
+  };
+
+  template<MethodType method_type, ExceptionCheck exception_check>
+  static void Get(JNIEnv* env,
+                  jclass clazz,
+                  const char* method_name,
+                  const char* jni_signature,
+                  jmethodID* method_id) {
+    subtle::AtomicWord* atomic = reinterpret_cast<subtle::AtomicWord*>(

[Sami, 2012/10/02 15:14:05]

I'm not sure which is better for this use, but an alternative would be to keep
the method id as a thread local value. Of course, there's some overhead to using
TLS too. For instance, every thread needs to redo the lookup.

[bulach, 2012/10/02 15:28:30]

hmm... we have base::internal::ThreadLocalPointer, but I presume the lookup /
set is more expensive than the atomic load / store?
of course, I can try to measure that too if you want.. :)

+        method_id);
+    subtle::AtomicWord value = base::subtle::Acquire_Load(atomic);
+    if (value)
+      return;
+    jmethodID id = method_type == METHODTYPE_STATIC ?
+        env->GetStaticMethodID(clazz, method_name, jni_signature) :
+        env->GetMethodID(clazz, method_name, jni_signature);
+    if (exception_check == EXCEPTIONCHECK_YES) {
+      CHECK(id || base::android::ClearException(env)) <<
+        "Failed to find " <<
+        (method_type == METHODTYPE_STATIC ? "static " : "") <<
+        "method " << method_name << " " << jni_signature;
+    } else if (base::android::HasException(env)) {
+      env->ExceptionClear();
+    }
+    base::subtle::Acquire_Store(
+        atomic, reinterpret_cast<subtle::AtomicWord>(id));
+  }
+};
+
+
 }  // namespace android
 }  // namespace base