// 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_SCOPED_JAVA_REF_H_
#define BASE_ANDROID_SCOPED_JAVA_REF_H_
#include <jni.h>
#include <stddef.h>
#include "base/base_export.h"
#include "base/basictypes.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 BASE_EXPORT JavaRef<jobject> {
public:
jobject obj() const { return obj_; }
bool is_null() const { return obj_ == NULL; }
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.
JNIEnv* SetNewLocalRef(JNIEnv* env, jobject obj);
void SetNewGlobalRef(JNIEnv* env, jobject obj);
void ResetLocalRef(JNIEnv* env);
void ResetGlobalRef();
jobject ReleaseInternal();
private:
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.
// Instances of this class may hold onto any JNIEnv passed into it until
// destroyed. Therefore, since a JNIEnv is only suitable for use on a single
// thread, objects of this class must be created, used, and destroyed, on a
// single thread.
// Therefore, this class should only be used as a stack-based object and from a
// single thread. If you wish to have the reference outlive the current
// callstack (e.g. as a class member) or you wish to pass it across threads,
// use a ScopedJavaGlobalRef instead.
template<typename T>
class ScopedJavaLocalRef : public JavaRef<T> {
public:
ScopedJavaLocalRef() : env_(NULL) {}
// Non-explicit copy constructor, to allow ScopedJavaLocalRef to be returned
// by value as this is the normal usage pattern.
ScopedJavaLocalRef(const ScopedJavaLocalRef<T>& other)
: env_(other.env_) {
this->SetNewLocalRef(env_, other.obj());
}
template<typename U>
explicit ScopedJavaLocalRef(const U& other)
: env_(NULL) {
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), env_(env) {}
~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->ResetLocalRef(env_);
}
template<typename U>
void Reset(const ScopedJavaLocalRef<U>& other) {
// We can copy over env_ here as |other| instance must be from the same
// thread as |this| local ref. (See class comment for multi-threading
// limitations, and alternatives).
this->Reset(other.env_, other.obj());
}
template<typename U>
void Reset(const U& other) {
// If |env_| was not yet set (is still NULL) it will be attached to the
// current thread in SetNewLocalRef().
this->Reset(env_, other.obj());
}
template<typename U>
void Reset(JNIEnv* env, U obj) {
implicit_cast<T>(obj); // Ensure U is assignable to T
env_ = 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());
}
private:
// This class is only good for use on the thread it was created on so
// it's safe to cache the non-threadsafe JNIEnv* inside this object.
JNIEnv* env_;
};
// Holds a global reference to a Java object. The global reference is scoped
// to the lifetime of this object. This class does not hold onto any JNIEnv*
// passed to it, hence it is safe to use across threads (within the constraints
// imposed by the underlying Java object that it references).
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->ResetGlobalRef();
}
template<typename U>
void Reset(const U& other) {
this->Reset(NULL, 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_