首先定位到com.android.launcher3.IconCache.java這個類的cacheLocked（）方法

private CacheEntry cacheLocked(ComponentName componentName, LauncherActivityInfoCompat info,
            HashMap<Object, CharSequence> labelCache, UserHandleCompat user, boolean usePackageIcon) {
        CacheKey cacheKey = new CacheKey(componentName, user);
        CacheEntry entry =
 
 mCache.get(cacheKey);
        if (entry == null) {
            entry = new CacheEntry();
        .......
                entry.contentDescription = mUserManager.getBadgedLabelForUser(entry.title, user);
                //用於初始化應用圖示
                entry.icon = Utilities.createIconBitmap(
                        info.
 
getBadgedIcon(mIconDpi), mContext);
            } else {
        ......
        return entry;
    }

首先通過通過info.getBadgedIcon(mIconDpi)來獲取應用原始launcher圖示的Drawable，mIconDpi為圖圖片資源id，然後通過Utilities.createIconBitmap（Drawable icon, Context context）方法來對圖示進行簡單的處理（要修改所有Launcher圖示的背景等，可以在此方法中修改）。

其中info.getBadgedIcon(mIconDpi)就是決定應用圖示的方法。該方法最終會進入com.android.launcher3.compat.LauncherActivityInfoCompatV16.java類的getBadgedIcon（）–>getIcon();通過Resources來獲取圖示的。 
我採用的方法，首先自定義一個類，LauncherIconTheme.java，該類的內容如下

package com.android.launcher3;

import com.android.launcher3.R;

import android.content.Context;
import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.util.Log;

public final  class LauncherIconTheme {
    //google郵箱
    private static int GMAIL = R.drawable.ic_launcher_gmail;
    //google地圖
    private static int GOOGLE_MAPS = R.drawable.ic_launcher_googlemaps;
    //自定義的應用資源id
    ...................

    private static String TAG = "LauncherIconTheme";

        //根據包名、類名獲取Bitmap
    public static Bitmap  getIconBitmap(Context context , String packageName , String className) {
        Resources resources = context.getResources();
        int iconId = getIconId(packageName, className);
        if (iconId != -1){
            return BitmapFactory.decodeResource(resources, iconId);
        }
        return null;
    }
        //根據包名、類名獲取Drawable   要用到的就是這個方法
    public static Drawable getIconDrawable(Context context , String packageName , String className) {
        Resources resources = context.getResources();
        int iconId = getIconId(packageName, className);
        if ( iconId != -1) {
            return resources.getDrawable(iconId);
        }
        return null;
    }
        //根據包名、類名獲取資源定義的圖示資源id
    private static int getIconId(String packageName , String className){
        if ( "com.google.android.gm".equals(packageName)
                && "com.google.android.gm.ConversationListActivityGmail".equals(className)) {
            return GMAIL;

        }else if ("com.google.android.apps.maps".equals(packageName)
                && "com.google.android.maps.MapsActivity".equals(className)) {
            return GOOGLE_MAPS;

        }else if ....................
            .................................
        }else{
            return -1;          
        }
    }   
}

然後在IconCache的cacheLocked方法中

    /**
     * Retrieves the entry from the cache. If the entry is not present, it creates a new entry.
     * This method is not thread safe, it must be called from a synchronized method.
     */
    private CacheEntry cacheLocked(ComponentName componentName, LauncherActivityInfoCompat info,
            HashMap<Object, CharSequence> labelCache, UserHandleCompat user, boolean usePackageIcon) {
        CacheKey cacheKey = new CacheKey(componentName, user);
        CacheEntry entry = mCache.get(cacheKey);
        if (entry == null) {
           .................................

                entry.contentDescription = mUserManager.getBadgedLabelForUser(entry.title, user);
                //實現修改圖示的邏輯
                Drawable themeDrawable = LauncherIconTheme
                        .getIconDrawable(mContext,
                                componentName.getPackageName(), componentName.getClassName());
                if ( themeDrawable != null ) {
                    entry.icon = Utilities.createIconBitmap(
                            themeDrawable, mContext);
                }else{
                    entry.icon = Utilities.createIconBitmap(
                            info.getBadgedIcon(mIconDpi), mContext);                    
                }
            } else {
             ...........................................
        return entry;
    }

結束。此方法中只是簡單的在Launcher中實現自定義應用圖示的功能，有啥不足希望大家指出，共同進步。