1.常用方法
public int getResId(String name,Context context){

    Resources r　＝　ｃｏｎｔｅｘｔ.getResources();

    int id = r.getIdentifier(name,"drawable","ｃｏｍ.demo");

    return id;

}

２．使用反射（推荐，性能高）
public class ResourceMan {

    public static int getResId(String variableName, Class<?> c) {

        try {

            Field idField = c.getDeclaredField(variableName);

            return idField.getInt(idField);

        } catch (Exception e) {

            e.printStackTrace();

            return -1;

        }

    }

}

int id = ResourceMan.getResId("icon",R.drawable.class);