改造错误：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法

Question

我正在尝试创建一个应用程序以允许用户注册和帐户。

我已经看到这个问题问了几次，但我无法使用相同的解决方案来解决我的问题

我收到了错误

/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.app, PID: 12181
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String okhttp3.ResponseBody.string()' on a null object reference
        at com.example.app.MainActivity$1.onResponse(MainActivity.java:75)
        at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run(ExecutorCallAdapterFactory.java:70)
        at android.os.Handler.handleCallback(Handler.java:883)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
I/Process: Sending signal. PID: 12181 SIG: 9
Process 12181 terminated.

我的改造客户是这样的

public class RetrofitClient {

    private static final String BASE_URL = "http://123.456.789.101:5000/api/";
    private static RetrofitClient mInstance;
    private Retrofit retrofit;

    private RetrofitClient(){
        retrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();
    }

    public static synchronized RetrofitClient getInstance() {

        if (mInstance == null) {
            mInstance = new RetrofitClient();
        }


        return mInstance;

    }

    public Api getApi(){
        return retrofit.create(Api.class);
    }
}

API接口是这样的

public interface Api {
    @FormUrlEncoded
    @POST("/users/")
    Call<ResponseBody> createUser(
            @Field("email") String email,
            @Field("password") String password

    );
}

主要活动是这个

Call<ResponseBody> call = RetrofitClient
                .getInstance()
                .getApi()
                .createUser(email, password);

        call.enqueue(new Callback<ResponseBody>() {
            @Override
            public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
                try {
                    String s = response.body().string();
                    Toast.makeText(MainActivity.this, s, Toast.LENGTH_LONG).show();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void onFailure(Call<ResponseBody> call, Throwable t) {
                Toast.makeText(MainActivity.this, t.getMessage(), Toast.LENGTH_LONG).show();

            }
        });

响应为空，所以我假设这是 RetrofitClient 的问题，但不确定如何修复

Answer 1

可能你的回复是空的，先检查一下。

if(response.body() != null) {
        String s = response.body().string();
    }