单例模式(Singleton) 2

Posted 2021-08-04 xhBruce

单例模式(Singleton) 2

Java设计模式 —— 单例模式(Singleton)

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Java

android 中 frameworks\\base\\core\\java\\android\\util\\Singleton.java

/*
 * Copyright (C) 2010 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.util;

import android.compat.annotation.UnsupportedAppUsage;

/**
 * Singleton helper class for lazily initialization.
 *
 * Modeled after frameworks/base/include/utils/Singleton.h
 *
 * @hide
 */
public abstract class Singleton<T> {

    @UnsupportedAppUsage
    public Singleton() {
    }

    @UnsupportedAppUsage
    private T mInstance;

    protected abstract T create();

    @UnsupportedAppUsage
    public final T get() {
        synchronized (this) {
            if (mInstance == null) {
                mInstance = create();
            }
            return mInstance;
        }
    }
}

用一下SingletonSampler

public class SingletonSampler {
    private String mTag = "SingletonSampler";

    private static final Singleton<SingletonSampler> mInstance = new Singleton<SingletonSampler>() {
        @Override
        protected SingletonSampler create() {
            return new SingletonSampler();
        }
    };

    public static SingletonSampler getInstance() {
        return mInstance.get();
    }

    private SingletonSampler() {
    }

    public void setLog(String tag) {
        mTag = tag;
    }

    @Override
    public String toString() {
        return getClass().getSimpleName() + "------" + mTag;
    }
}

public class Main {
    public static void main(String[] args) {
        SingletonSampler sampler1 = SingletonSampler.getInstance();
        SingletonSampler sampler2 = SingletonSampler.getInstance();

        System.out.println(sampler2.toString());
        sampler1.setLog("xhbruce");
        System.out.println(sampler2.toString());
    }
}

C/C++

Android 中 system\\core\\libutils\\include\\utils\\Singleton.h

/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef ANDROID_UTILS_SINGLETON_H
#define ANDROID_UTILS_SINGLETON_H

#include <stdint.h>

// some vendor code assumes they have atoi() after including this file.
#include <stdlib.h>

#include <sys/types.h>
#include <utils/Mutex.h>
#include <cutils/compiler.h>

namespace android {
// ---------------------------------------------------------------------------

// Singleton<TYPE> may be used in multiple libraries, only one of which should
// define the static member variables using ANDROID_SINGLETON_STATIC_INSTANCE.
// Turn off -Wundefined-var-template so other users don't get:
// instantiation of variable 'android::Singleton<TYPE>::sLock' required here,
// but no definition is available
#if defined(__clang__)
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wundefined-var-template"
#endif

// DO NOT USE: Please use scoped static initialization. For instance:
//     MyClass& getInstance() {
//         static MyClass gInstance(...);
//         return gInstance;
//     }
template <typename TYPE>
class ANDROID_API Singleton
{
public:
    static TYPE& getInstance() {
        Mutex::Autolock _l(sLock);
        TYPE* instance = sInstance;
        if (instance == nullptr) {
            instance = new TYPE();
            sInstance = instance;
        }
        return *instance;
    }

    static bool hasInstance() {
        Mutex::Autolock _l(sLock);
        return sInstance != nullptr;
    }
    
protected:
    ~Singleton() { }
    Singleton() { }

private:
    Singleton(const Singleton&);
    Singleton& operator = (const Singleton&);
    static Mutex sLock;
    static TYPE* sInstance;
};

#if defined(__clang__)
#pragma clang diagnostic pop
#endif

/*
 * use ANDROID_SINGLETON_STATIC_INSTANCE(TYPE) in your implementation file
 * (eg: <TYPE>.cpp) to create the static instance of Singleton<>'s attributes,
 * and avoid to have a copy of them in each compilation units Singleton<TYPE>
 * is used.
 * NOTE: we use a version of Mutex ctor that takes a parameter, because
 * for some unknown reason using the default ctor doesn't emit the variable!
 */

#define ANDROID_SINGLETON_STATIC_INSTANCE(TYPE)                 \\
    template<> ::android::Mutex  \\
        (::android::Singleton< TYPE >::sLock)(::android::Mutex::PRIVATE);  \\
    template<> TYPE* ::android::Singleton< TYPE >::sInstance(nullptr);  /* NOLINT */ \\
    template class ::android::Singleton< TYPE >;


// ---------------------------------------------------------------------------
}  // namespace android

#endif // ANDROID_UTILS_SINGLETON_H

用一下SingletonSampler