Retrofit+RxJava实战日志(3)-网络异常处理

上一篇文章写了Retrofit和RxJava的基本使用，最后的subscriber并没有写具体实现
成功的返回通常对应了各种不同的处理，而异常返回通常有着统一的处理
因此这一篇单独讲讲异常处理

Retrofit本身会抛出HttpException，Gson解析会抛出解析异常，
此外我们还应该处理与服务器约定好的“异常”，即上一篇提到的返回数据中result字段值不会0的情况

这里要先解决一个问题，就是Gson构建的对象，通过注解定义key名，以变量的类型定value的类型，
但如果同样的key在不同情况下属于不同的数据类型，就会出问题。
比如上一篇文章提及，服务器返回格式是

{
    "result":"结果代号，0表示成功",
    "msg":"成功返回时是消息数据列表，失败时是异常消息文本"
}

那么msg究竟应该定义为String，还是一个List呢

我找到的解决方法就是：
注册一个自定义的转换类GsonResponseBodyConverter
先用一个只含result变量的Model类去解析获得result值
如果失败，则用msg是String的Model类再去解析msg值，然后组成一个ResultException
如果成功，则按照原本的Model类解析
代码如下：

class GsonResponseBodyConverter<T> implements Converter<ResponseBody, T> {
    private final Gson gson;
    private final Type type;

    GsonResponseBodyConverter(Gson gson, Type type) {
        this.gson = gson;
        this.type = type;
    }

    @Override
    public T convert(ResponseBody value) throws IOException{
        String response = value.string();
        try {
            Log.d("Network", "response>>" + response);
            //ResultResponse 只解析result字段
            ResultResponse resultResponse = gson.fromJson(response, ResultResponse.class);
            if (resultResponse.getResult() == 0){
                //result==0表示成功返回，继续用本来的Model类解析
                return gson.fromJson(response, type);
            } else {
                //ErrResponse 将msg解析为异常消息文本
                ErrResponse errResponse = gson.fromJson(response, ErrResponse.class);
                throw new ResultException(resultResponse.getResult(), errResponse.getMsg());
            }
        } finally {
        }
    }
}

这个类用于捕获服务器约定的错误类型

public class ResultException extends RuntimeException {

    private int errCode = 0;

    public ResultException(int errCode, String msg) {
        super(msg);
        this.errCode = errCode;
    }

    public int getErrCode() {
        return errCode;
    }
}

拷贝原生的ResponseConverterFactory，将GsonResponseBodyConverter替换为前面我们自定义的

public class ResponseConverterFactory extends Converter.Factory {

    ...
    ...

    @Override
    public Converter fromResponseBody(Type type, Annotation[] annotations) {
        return new GsonResponseBodyConverter<>(gson, type);
    }

    @Override
    public Converter toRequestBody(Type type, Annotation[] annotations) {
        return new GsonRequestBodyConverter<>(gson, type);
    }

}

然后在构建Retrofit时注册这个工厂类

Retrofit = new Retrofit.Builder()
                .baseUrl(API_SERVER + "/")
                //注册自定义的工厂类
                .addConverterFactory(ResponseConverterFactory.create())
                .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
                .client(mOkHttpClient)
                .build();

这样就完成了Retrofit也可以抛出服务器约定异常

然后就是具体的处理：

public abstract class AbsAPICallback<T> extends Subscriber<T> {

    //对应HTTP的状态码
    private static final int UNAUTHORIZED = 401;
    private static final int FORBIDDEN = 403;
    private static final int NOT_FOUND = 404;
    private static final int REQUEST_TIMEOUT = 408;
    private static final int INTERNAL_SERVER_ERROR = 500;
    private static final int BAD_GATEWAY = 502;
    private static final int SERVICE_UNAVAILABLE = 503;
    private static final int GATEWAY_TIMEOUT = 504;
    //出错提示
    private final String networkMsg;
    private final String parseMsg;
    private final String unknownMsg;

    protected AbsAPICallback(String networkMsg, String parseMsg, String unknownMsg) {
        this.networkMsg = networkMsg;
        this.parseMsg = parseMsg;
        this.unknownMsg = unknownMsg;
    }


    @Override
    public void onError(Throwable e) {
        Throwable throwable = e;
        //获取最根源的异常
        while(throwable.getCause() != null){
            e = throwable;
            throwable = throwable.getCause();
        }

        ApiException ex;
        if (e instanceof HttpException){             //HTTP错误
            HttpException httpException = (HttpException) e;
            ex = new ApiException(e, httpException.code());
            switch(httpException.code()){
                case UNAUTHORIZED:
                case FORBIDDEN:
                    onPermissionError(ex);          //权限错误，需要实现
                    break;
                case NOT_FOUND:
                case REQUEST_TIMEOUT:
                case GATEWAY_TIMEOUT:
                case INTERNAL_SERVER_ERROR:
                case BAD_GATEWAY:
                case SERVICE_UNAVAILABLE:
                default:
                    ex.setDisplayMessage(networkMsg);  //均视为网络错误
                    onError(ex);
                    break;
            }
        } else if (e instanceof ResultException){    //服务器返回的错误
            ResultException resultException = (ResultException) e;
            ex = new ApiException(resultException, resultException.getErrCode());               
            onResultError(ex);  
        } else if (e instanceof JsonParseException
                || e instanceof JSONException
                || e instanceof ParseException){
            ex = new ApiException(e, ApiException.PARSE_ERROR);
            ex.setDisplayMessage(parseMsg);            //均视为解析错误
            onError(ex);
        } else {
            ex = new ApiException(e, ApiException.UNKNOWN);
            ex.setDisplayMessage(unknownMsg);          //未知错误
            onError(ex);
        }
    }


    /**
     * 错误回调
     */
    protected abstract void onError(ApiException ex);

    /**
     * 权限错误，需要实现重新登录操作
     */
    protected abstract void onPermissionError(ApiException ex);

    /**
     * 服务器返回的错误
     */
    protected abstract void onResultError(ApiException ex);

    @Override
    public void onCompleted() {

    }

}

自定义ApiException，携带了异常代码和信息，以及根源Throwable，足够调用者需要

public class ApiException extends Exception {

    private final int code;
    private String displayMessage;

    public static final int UNKNOWN = 1000;
    public static final int PARSE_ERROR = 1001;

    public ApiException(Throwable throwable, int code) {
        super(throwable);
        this.code = code;
    }

    public int getCode() {
        return code;
    }
    public String getDisplayMessage() {
        return displayMessage;
    }
    public void setDisplayMessage(String msg) {
        this.displayMessage = msg + "(code:" + code + ")";
    }
}