Android Application not installed

 今天在做软件自动升级的的过程中，点击从服务器下载下来的apk，调用系统程序进行安装的过程中 系统提示 Application not installed 

/**
     *
     * install
     * @description: TODO(软件安装)
     * @param fileName
     * @since   2012-2-9    liaoyp
     */
    public void install(String fileName){
        File    file2 = new File(fileName + "JCSClient.apk");
        System.out.println(fileName+"JCSClient.apk");
        Intent intent = new Intent();
        String   type = "application/vnd.android.package-archive";
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        intent.setAction(Intent.ACTION_VIEW);
        System.out.println("Uri"+Uri.fromFile(file2));
        intent.setDataAndType(Uri.fromFile(file2), type);
        startActivity(intent);

      //Preferences.getInstance(LoadingActivity.this).edit().clear().commit();
        LoadingActivity.this.finish();
        System.exit(0);
    }

不知什么原因，有哪位大侠帮忙解释下？ 同未发下logcat中出现异常情况