Refactor ScopedJavaRef

base/android/scoped_java_ref.h

+// Copyright (c) 2011 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_SCOPED_JAVA_REF_H_
+#define BASE_ANDROID_SCOPED_JAVA_REF_H_
+
+#include <jni.h>
+#include <stddef.h>
+
+#include "base/logging.h"
+
+namespace base {
+namespace android {
+
+// Forward declare the generic java reference template class.
+template<typename T> class JavaRef;
+
+// Template specialization of JavaRef, which acts as the base class for all
+// other JavaRef<> template types. This allows you to e.g. pass
+// ScopedJavaLocalRef<jstring> into a function taking const JavaRef<jobject>&
+template<>
+class JavaRef<jobject> {
+ public:
+  JNIEnv* env() const { return env_; }
+  jobject obj() const { return obj_; }
+
+ protected:
+  // Initializes a NULL reference.
+  JavaRef();
+
+  // Takes ownership of the |obj| reference passed; requires it to be a local
+  // reference type.
+  JavaRef(JNIEnv* env, jobject obj);
+
+  ~JavaRef();
+
+  // The following are implementation detail convenience methods, for
+  // use by the sub-classes.
+  void SetNewLocalRef(JNIEnv* env, jobject obj);
+  void SetNewGlobalRef(JNIEnv* env, jobject obj);
+  jobject ReleaseInternal();
+
+ private:
+  JNIEnv* env_;
+  jobject obj_;
+
+  DISALLOW_COPY_AND_ASSIGN(JavaRef);
+};
+
+// Generic base class for ScopedJavaLocalRef and ScopedJavaGlobalRef. Useful
+// for allowing functions to accept a reference without having to mandate
+// whether it is a local or global type.
+template<typename T>
+class JavaRef : public JavaRef<jobject> {
+ public:
+  T obj() const { return static_cast<T>(JavaRef<jobject>::obj()); }
+
+ protected:
+  JavaRef() {}
+  ~JavaRef() {}
+
+  JavaRef(JNIEnv* env, T obj) : JavaRef<jobject>(env, obj) {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(JavaRef);
+};
+
+// Holds a local reference to a Java object. The local reference is scoped
+// to the lifetime of this object. Note that since a JNI Env is only suitable
+// for use on a single thread, objects of this class must be created, used and
+// destroyed on the same thread.
+// In general, this class should only be used as a stack-based object. If you
+// wish to have the reference outlive the current callstack (e.g. as a class
+// member) use ScopedJavaGlobalRef instead.
+template<typename T>
+class ScopedJavaLocalRef : public JavaRef<T> {
+ public:
+  ScopedJavaLocalRef() {}
+
+  // Non-explicit copy constructor, to allow ScopedJavaLocalRef to be returned
+  // by value as this is the normal usage pattern.
+  ScopedJavaLocalRef(const ScopedJavaLocalRef<T>& other) {
+    this->Reset(other);
+  }
+
+  template<typename U>
+  explicit ScopedJavaLocalRef(const U& other) {
+    this->Reset(other);
+  }
+
+  // Assumes that |obj| is a local reference to a Java object and takes
+  // ownership  of this local reference.
+  ScopedJavaLocalRef(JNIEnv* env, T obj) : JavaRef<T>(env, obj) {}
+
+  ~ScopedJavaLocalRef() {
+    this->Reset();
+  }
+
+  // Overloaded assignment operator defined for consistency with the implicit
+  // copy constructor.
+  void operator=(const ScopedJavaLocalRef<T>& other) {
+    this->Reset(other);
+  }
+
+  void Reset() {
+    this->SetNewLocalRef(NULL, NULL);
+  }
+
+  template<typename U>
+  void Reset(const U& other) {
+    this->Reset(other.env(), other.obj());
+  }
+
+  template<typename U>
+  void Reset(JNIEnv* env, U obj) {
+    implicit_cast<T>(obj);  // Ensure U is assignable to T
+    this->SetNewLocalRef(env, obj);
+  }
+
+  // Releases the local reference to the caller. The caller *must* delete the
+  // local reference when it is done with it.
+  T Release() {
+    return static_cast<T>(this->ReleaseInternal());
+  }
+};
+
+// Holds a global reference to a Java object. The global reference is scoped
+// to the lifetime of this object. Note that since a JNI Env is only suitable
+// for use on a single thread, objects of this class must be created, used and
+// destroyed on the same thread.
+template<typename T>
+class ScopedJavaGlobalRef : public JavaRef<T> {
+ public:
+  ScopedJavaGlobalRef() {}
+
+  explicit ScopedJavaGlobalRef(const ScopedJavaGlobalRef<T>& other) {
+    this->Reset(other);
+  }
+
+  template<typename U>
+  explicit ScopedJavaGlobalRef(const U& other) {
+    this->Reset(other);
+  }
+
+  ~ScopedJavaGlobalRef() {
+    this->Reset();
+  }
+
+  void Reset() {
+    this->SetNewGlobalRef(NULL, NULL);
+  }
+
+  template<typename U>
+  void Reset(const U& other) {
+    this->Reset(other.env(), other.obj());
+  }
+
+  template<typename U>
+  void Reset(JNIEnv* env, U obj) {
+    implicit_cast<T>(obj);  // Ensure U is assignable to T
+    this->SetNewGlobalRef(env, obj);
+  }
+
+  // Releases the global reference to the caller. The caller *must* delete the
+  // global reference when it is done with it.
+  T Release() {
+    return static_cast<T>(this->ReleaseInternal());
+  }
+};
+
+}  // namespace android
+}  // namespace base
+
+#endif  // BASE_ANDROID_SCOPED_JAVA_REF_H_