如何在Flutter应用中通过原生Java SDK调用Brother打印机打印标签？

你遇到的android.os.NetworkOnMainThreadException是Android系统的强制限制：所有网络通信类操作禁止在UI主线程执行。当前Brother SDK的打印机连接、打印逻辑直接运行在Flutter MethodChannel的默认回调线程（即Android主线程），触发了系统拦截。

1. 调整Java层打印逻辑到工作线程执行 #

将打印逻辑移到独立工作线程中运行，执行完成后切回主线程回调结果给Flutter，修改后的Java层代码如下：

if (call.method.equals("print")) {
    String address = call.argument("address");
    String base64 = call.argument("bas64");
    final Result finalResult = result;

    // 启动工作线程执行网络相关操作
    new Thread(() -> {
        Printer printer = new Printer();
        PrinterInfo settings = printer.getPrinterInfo();
        settings.printerModel = PrinterInfo.Model.QL_810W;
        settings.port = PrinterInfo.Port.NET;
        settings.ipAddress = address;

        // 打印参数配置和原有逻辑保持一致
        settings.paperSize = PrinterInfo.PaperSize.CUSTOM;
        settings.orientation = PrinterInfo.Orientation.LANDSCAPE;
        settings.valign = PrinterInfo.VAlign.MIDDLE;
        settings.align = PrinterInfo.Align.CENTER;
        settings.printMode = PrinterInfo.PrintMode.FIT_TO_PAGE;
        settings.numberOfCopies = 1;

        settings.labelNameIndex = LabelInfo.QL700.W52H29.ordinal();
        //settings.labelNameIndex = LabelInfo.QL700.W62.ordinal();

        settings.isAutoCut = true;
        settings.isCutAtEnd = false;
        settings.isHalfCut = false;
        settings.isSpecialTape = false;

        printer.setPrinterInfo(settings);
        PrinterStatus bit = new PrinterStatus();

        if (printer.startCommunication()) {
            bit = printer.printImage(bmpFromBase64(base64));
            printer.endCommunication();
            
            // MethodChannel的结果回调必须在主线程执行
            new Handler(Looper.getMainLooper()).post(() -> {
                if (bit.errorCode != PrinterInfo.ErrorCode.ERROR_NONE) {
                    Log.d("TAG", "ERROR - " + bit.errorCode);
                    finalResult.error("ERROR",bit.errorCode.toString(),"Error when Printing");
                } else {
                    finalResult.success(0);
                }
            });
        } else {
            new Handler(Looper.getMainLooper()).post(() -> 
                finalResult.error("ERROR", "CONNECTION_FAILED", "Failed to connect printer")
            );
        }
    }).start();
}else{
    result.notImplemented();
}

2. 配套检查项 #

• 确认AndroidManifest.xml中已添加必要权限：

<!-- 网络通信权限 -->
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

• 若你的应用适配Android 9及以上系统，打印机为内网IP使用明文Socket通信的话，需要在AndroidManifest.xml的application标签添加android:usesCleartextTraffic="true"。
• 你当前传参使用的key为bas64，只要Flutter端和Java端保持一致即可正常取值，若后续出现空图异常可优先核对该参数。

内容的提问来源于stack exchange，提问作者Abinandhan Selvaraj