Android: lazy initialization for method id.

base/android/jni_android.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_ANDROID_JNI_ANDROID_H_
 #define BASE_ANDROID_JNI_ANDROID_H_
 
 #include <jni.h>
 #include <sys/types.h>
 
 #include "base/android/scoped_java_ref.h"
+#include "base/atomicops.h"
 #include "base/compiler_specific.h"
 
 namespace base {
 namespace android {
 
 // Used to mark symbols to be exported in a shared library's symbol table.
 #define JNI_EXPORT __attribute__ ((visibility("default")))
 
 // Contains the registration method information for initializing JNI bindings.
 struct RegistrationMethod {
@@ skipping 28 matching lines @@
 // Use HasClass if you need to check whether the class exists.
 ScopedJavaLocalRef<jclass> GetClass(JNIEnv* env, const char* class_name);
 
 // Similar to the above, but the caller is responsible to manage the jclass
 // lifetime.
 jclass GetUnscopedClass(JNIEnv* env, const char* class_name) WARN_UNUSED_RESULT;
 
 // Returns true iff the class |class_name| could be found.
 bool HasClass(JNIEnv* env, const char* class_name);
 
-// Returns the method ID for the method with the specified name and signature.
-// This method triggers a fatal assertion if the method could not be found.
-// Use HasMethod if you need to check whether a method exists.
-jmethodID GetMethodID(JNIEnv* env,
-                      const JavaRef<jclass>& clazz,
-                      const char* method_name,
-                      const char* jni_signature);
+// This class is a wrapper for JNIEnv Get(Static)MethodID.
+class MethodID {
+ public:
+  enum MethodType {
+    METHODTYPE_STATIC,
+    METHODTYPE_NORMAL,
+  };
 
-// Similar to GetMethodID, but takes a raw jclass.
-jmethodID GetMethodID(JNIEnv* env,
-                      jclass clazz,
-                      const char* method_name,
-                      const char* jni_signature);
+  // Wether or not to CHECK if an exception happens when
+  // trying to obtain a method id.
+  enum ExceptionCheck {
+    EXCEPTIONCHECK_NO,
+    EXCEPTIONCHECK_YES,
+  };
 
-// Unlike GetMethodID, returns NULL if the method could not be found.
-jmethodID GetMethodIDOrNull(JNIEnv* env,
-                            jclass clazz,
-                            const char* method_name,
-                            const char* jni_signature);
+  template<MethodType method_type,
+           ExceptionCheck exception_check>
+  static jmethodID Get(JNIEnv* env,
+                       jclass clazz,
+                       const char* method_name,
+                       const char* jni_signature);
 
-// Returns the method ID for the static method with the specified name and
-// signature.
-// This method triggers a fatal assertion if the method could not be found.
-// Use HasMethod if you need to check whether a method exists.
-jmethodID GetStaticMethodID(JNIEnv* env,
-                            const JavaRef<jclass>& clazz,
-                            const char* method_name,
-                            const char* jni_signature);
-
-// Similar to the GetStaticMethodID, but takes a raw jclass.
-jmethodID GetStaticMethodID(JNIEnv* env,
-                            jclass clazz,
-                            const char* method_name,
-                            const char* jni_signature);
-
-// Unlike GetStaticMethodID, returns NULL if the method could not be found.
-jmethodID GetStaticMethodIDOrNull(JNIEnv* env,
-                                  jclass clazz,
-                                  const char* method_name,
-                                  const char* jni_signature);
-
-// Returns true iff |clazz| has a method with the specified name and signature.
-bool HasMethod(JNIEnv* env,
-               const JavaRef<jclass>& clazz,
-               const char* method_name,
-               const char* jni_signature);
+  // The caller is responsible to zero-initialize |atomic_method_id|.
+  // If it's set, it'll return immediately. Otherwise, it'll call into
+  // ::Get() above. If there's a race, it's ok since the values are the same

[joth, 2012/10/04 17:59:59]

nit: if there's a race... is all implementation detail, moving into the .cc as
not needed on the API. (could just call out here that it's fine to
simultaneously call this on multiple threads referencing the same
|atomic_method_id|

[bulach, 2012/10/04 18:58:17]

Done.

+  // (and the duplicated effort will happen only once).
+  template<MethodType method_type,
+           ExceptionCheck exception_check>
+  static jmethodID LazyGet(JNIEnv* env,
+                           jclass clazz,
+                           const char* method_name,
+                           const char* jni_signature,
+                           base::subtle::AtomicWord* atomic_method_id);
+};
 
 // Gets the method ID from the class name. Clears the pending Java exception
 // and returns NULL if the method is not found. Caches results. Note that
-// GetMethodID() below avoids a class lookup, but does not cache results.
+// MethodID() above avoids a class lookup, but does not cache results.

[joth, 2012/10/04 17:59:59]

nit: MethodID above is not a function. MethodID::Get()

[bulach, 2012/10/04 18:58:17]

Done.

 // Strings passed to this function are held in the cache and MUST remain valid
 // beyond the duration of all future calls to this function, across all
 // threads. In practice, this means that the function should only be used with
 // string constants.
 jmethodID GetMethodIDFromClassName(JNIEnv* env,
                                    const char* class_name,
                                    const char* method,
                                    const char* jni_signature);
 
 // Gets the field ID for a class field.
@@ skipping 24 matching lines @@
 // and returns true.
 bool ClearException(JNIEnv* env);
 
 // This function will call CHECK() macro if there's any pending exception.
 void CheckException(JNIEnv* env);
 
 }  // namespace android
 }  // namespace base
 
 #endif  // BASE_ANDROID_JNI_ANDROID_H_