做开发的朋友肯定对单例模式不陌生，大概有下面两种实现方式。

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public class Singleton{

    private static Singleton instance = null;

    private Singleton(){

    }

    public static synchronized Singleton getInstance(){
        if(instance == null){
            instance = new Singleton();
        }
        return instance;
    }

}
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public class Singleton{

    private static Singleton instance=new Singleton();

    private Singleton(){

    }

    public static Singletonget Instance(){
        return instance;
    }

}

但是在我们 Android 中，经常会使用到 Context 的引用，于是你写下下面这种单例

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public class Singleton{

    private static Singleton instance = null;

    private Singleton(Context ctx){

    }
    // Context 参数
    public static synchronized Singleton getInstance(Context ctx){
        if(instance == null){
            instance = new Singleton(ctx);
        }
        return instance;
    }

}
  • 每次获取单例的时候都要传入一个 Context 对象，如果我们要在纯 Java 类下使用，岂不是就歇菜了？
  • 所以正确的单例模式应该是这样的，以 PreferenceUtils 为例
  • 我们只需要在 Application 里面调用 createInstance(Context ctx)，在其他地方使用的时候只需要 getInstance()即可
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package cn.gavinliu.formater.util;

import java.util.Map.Entry;

import android.content.ContentValues;
import android.content.Context;
import android.content.SharedPreferences;

public class PreferenceUtils {

    private static PreferenceUtils util;
    private SharedPreferences mPreference;

    public static void createInstance(Context ctx) {
        util = new PreferenceUtils(ctx);
    }

    public static PreferenceUtils getInstance() {
        return util;
    }

    private PreferenceUtils(Context ctx) {
        mPreference = ctx.getApplicationContext().getSharedPreferences(ctx.getPackageName() + "_preferences", Context.MODE_PRIVATE);
    }

    public boolean putString(String key, String value) {
        return mPreference.edit().putString(key, value).commit();
    }

    public boolean putInt(String key, int value) {
        return mPreference.edit().putInt(key, value).commit();
    }

    public boolean putBoolean(String key, boolean value) {
        return mPreference.edit().putBoolean(key, value).commit();
    }

    public boolean putValues(ContentValues values) {
        SharedPreferences.Editor editor = mPreference.edit();
        for (Entry<String, Object> value : values.valueSet()) {
            editor.putString(value.getKey(), value.getValue().toString());
        }
        return editor.commit();
    }

    public String getString(String key) {
        return getString(key, "");
    }

    public String getString(String key, String defValue) {
        return mPreference.getString(key, defValue);
    }

    public int getInt(String key) {
        return getInt(key, -1);
    }

    public int getInt(String key, int defValue) {
        return mPreference.getInt(key, defValue);
    }

    public boolean getBoolean(String key) {
        return getBoolean(key, false);
    }

    public boolean getBoolean(String key, boolean defValue) {
        return mPreference.getBoolean(key, defValue);
    }
}