Java转Kotlin后调用SPUtil.putString触发空指针异常求助

原始Java代码 #

findViewById(R.id.registerButton).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                User user = new User(
                        idText.getText().toString(),
                        passwordText.getText().toString(),
                        emailText.getText().toString(),
                        sex,
                        organization);
                Gson gson = new Gson();
                String json = gson.toJson(user);
                SPUtil.putString(idText.getText().toString(), json);
                finish();
            }
        });

改写后的Kotlin代码 #

binding.registerButton.setOnClickListener {

            // spinner

            val genderGroup = findViewById<RadioGroup>(R.id.genderGroup)
            val genderGroupID = genderGroup.checkedRadioButtonId
            sex = findViewById<RadioButton>(genderGroupID).text.toString()
            val organization = spinner.selectedItem.toString()

            println("run this")

            var isGoToJoin = true

            val email = binding.idText.text.toString()
            val password1 = binding.passwordText.text.toString()
            val password2 = binding.passwordText2.text.toString()
            val sex = sex

//            println(email)
//            println(password1)
//            println(sex)
//            println(organization)


            // user 여기가 도저히 안된당
            val idText = email
            val passwordText = password1

            val user = User(
                idText,
                passwordText,
                sex,
                organization
            )

            val gson = Gson()
            val json = gson.toJson(user)

            println(json)
            SPUtil.putString(idText, json)

期望解析存储数据的代码 #

val user = Gson().fromJson(
            intent.getStringExtra("id"),
            User::class.java // User추가
        )
        println("homescreen")
        println(user)
        if (user!=null){
            findViewById<TextView>(R.id.user_id).text = user.id
            findViewById<TextView>(R.id.sex_id).text = user.sex
            findViewById<TextView>(R.id.organization_id).text = user.organization
            println(user)
        }

异常信息 #

FATAL EXCEPTION: main
Process: com.example.schoolboard, PID: 9955
java.lang.ExceptionInInitializerError
at com.example.schoolboard.utils.SPUtil.getInstance(SPUtil.java:25)
at com.example.schoolboard.utils.SPUtil.putString(SPUtil.java:33)
at com.example.schoolboard.auth.JoinActivity.onCreate$lambda$2(JoinActivity.kt:127)
at com.example.schoolboard.auth.JoinActivity.$r8$lambda$DLVZYaVRDjPwwdYMhAYrW_DySaA(Unknown Source:0)
at com.example.schoolboard.auth.JoinActivity$$ExternalSyntheticLambda0.onClick(Unknown Source:4)
at android.view.View.performClick(View.java:6256)
at android.view.View$PerformClick.run(View.java:24701)
at android.os.Handler.handleCallback(Handler.java:789)
at android.os.Handler.dispatchMessage(Handler.java:98)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:537)
at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:526)
at com.example.schoolboard.utils.SPUtil.<init>(SPUtil.java:17)
at com.example.schoolboard.utils.SPUtil$SingletonHolder.<init>(SPUtil.java:21)
at com.example.schoolboard.utils.SPUtil.getInstance(SPUtil.java:25) 
at com.example.schoolboard.utils.SPUtil.putString(SPUtil.java:33) 
at com.example.schoolboard.auth.JoinActivity.onCreate$lambda$2(JoinActivity.kt:127) 
at com.example.schoolboard.auth.JoinActivity.$r8$lambda$DLVZYaVRDjPwwdYMhAYrW_DySaA(Unknown Source:0) 
at com.example.schoolboard.auth.JoinActivity$$ExternalSyntheticLambda0.onClick(Unknown Source:4) 
at android.view.View.performClick(View.java:6256) 
at android.view.View$PerformClick.run(View.java:24701) 
at android.os.Handler.handleCallback(Handler.java:789) 
at android.os.Handler.dispatchMessage(Handler.java:98) 
at android.os.Looper.loop(Looper.java:164) 
at android.app.ActivityThread.main(ActivityThread.java:6541) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) 

问题分析与解决方案 #

从异常栈可以明确，问题根源是SPUtil初始化时调用PreferenceManager.getDefaultSharedPreferences()传入的Context为null，导致触发空指针异常。

核心修复步骤： #

1. 修正SPUtil的单例初始化逻辑
   确保SPUtil在获取单例时接收有效的Context，并使用Application Context避免内存泄漏：

   public class SPUtil {
       private static SPUtil instance;
       private SharedPreferences sp;
   
       private SPUtil(Context context) {
           // 使用Application Context确保生命周期安全
           sp = PreferenceManager.getDefaultSharedPreferences(context.getApplicationContext());
       }
   
       public static synchronized SPUtil getInstance(Context context) {
           if (instance == null) {
               instance = new SPUtil(context);
           }
           return instance;
       }
   
       public void putString(String key, String value) {
           sp.edit().putString(key, value).apply();
       }
   
       // 新增getString方法用于后续解析
       public String getString(String key, String defaultValue) {
           return sp.getString(key, defaultValue);
       }
   }
   

2. 修改Kotlin中SPUtil的调用方式
   在注册按钮的点击事件中，传入当前Activity的Context获取SPUtil实例：

   // 替换原有的SPUtil.putString(idText, json)
   SPUtil.getInstance(this).putString(idText, json)
   // 存储完成后记得调用finish()回到首页
   finish()
   

3. 修正首页的用户数据解析逻辑
   原代码直接用intent传参解析是错误的，需要从SP中取出对应key的JSON字符串再解析：

   val userId = intent.getStringExtra("id")
   val userJson = SPUtil.getInstance(this).getString(userId, null)
   val user = Gson().fromJson(userJson, User::class.java)
   
   println("homescreen")
   println(user)
   if (user != null){
       findViewById<TextView>(R.id.user_id).text = user.id
       findViewById<TextView>(R.id.sex_id).text = user.sex
       findViewById<TextView>(R.id.organization_id).text = user.organization
       println(user)
   }
   

内容的提问来源于stack exchange，提问作者BlancShon