Java模块包访问异常：Retrofit Gson转换JSON模型失败

使用Retrofit 2对接自研API，通过Retrofit Gson将API响应转换为实体对象。存放JSON模型类的包需要Gson访问，但添加开放该包的配置时构建报错；开放上级被javafx.fxml正常访问的包后构建没问题，但运行时Gson抛出如下错误：

Exception in thread "DefaultDispatcher-worker-2" java.lang.IllegalArgumentException: Unable to create converter for class fr.plaglefleau.banksystemdesktop.api.models.bankapi.response.get.LoginResponse
    for method BankApi.login
    at retrofit2@2.11.0/retrofit2.Utils.methodError(Utils.java:56)
    at retrofit2@2.11.0/retrofit2.HttpServiceMethod.createResponseConverter(HttpServiceMethod.java:139)
    at retrofit2@2.11.0/retrofit2.HttpServiceMethod.parseAnnotations(HttpServiceMethod.java:97)
    at retrofit2@2.11.0/retrofit2.ServiceMethod.parseAnnotations(ServiceMethod.java:39)
    at retrofit2@2.11.0/retrofit2.Retrofit.loadServiceMethod(Retrofit.java:235)
    at retrofit2@2.11.0/retrofit2.Retrofit$1.invoke(Retrofit.java:177)
    at jdk.proxy2/com.sun.proxy.jdk.proxy2.$Proxy6.login(Unknown Source)
    at fr.plaglefleau.banksystemdesktop@1.0-SNAPSHOT/fr.plaglefleau.banksystemdesktop.SignInController$signIn$1.invokeSuspend(SignInController.kt:95)
    at kotlin.stdlib/kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
    at kotlinx.coroutines.core/kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:108)
    at kotlinx.coroutines.core/kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:589)
    at kotlinx.coroutines.core/kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:806)
    at kotlinx.coroutines.core/kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:710)
    at kotlinx.coroutines.core/kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:697)
    Suppressed: kotlinx.coroutines.internal.DiagnosticCoroutineContextException: [StandaloneCoroutine{Cancelling}@5216989, Dispatchers.Default]
Caused by: com.google.gson.JsonIOException: Failed making field 'fr.plaglefleau.banksystemdesktop.api.models.bankapi.response.get.LoginResponse#message' accessible; either increase its visibility or write a custom TypeAdapter for its declaring type.
    at com.google.gson@2.10.1/com.google.gson.internal.reflect.ReflectionHelper.makeAccessible(ReflectionHelper.java:38)
    at com.google.gson@2.10.1/com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:286)
    at com.google.gson@2.10.1/com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:130)
Caused by: com.google.gson.JsonIOException: Failed making field 'fr.plaglefleau.banksystemdesktop.api.models.bankapi.response.get.LoginResponse#message' accessible; either increase its visibility or write a custom TypeAdapter for its declaring type.

    at com.google.gson@2.10.1/com.google.gson.Gson.getAdapter(Gson.java:556)
    at retrofit2.converter.gson@2.11.0/retrofit2.converter.gson.GsonConverterFactory.responseBodyConverter(GsonConverterFactory.java:64)
    at retrofit2@2.11.0/retrofit2.Retrofit.nextResponseBodyConverter(Retrofit.java:418)
    at retrofit2@2.11.0/retrofit2.Retrofit.responseBodyConverter(Retrofit.java:401)
    at retrofit2@2.11.0/retrofit2.HttpServiceMethod.createResponseConverter(HttpServiceMethod.java:137)
    ... 12 more
Caused by: java.lang.reflect.InaccessibleObjectException: Unable to make field private final java.lang.String fr.plaglefleau.banksystemdesktop.api.models.bankapi.response.get.LoginResponse.message accessible: module fr.plaglefleau.banksystemdesktop does not "opens fr.plaglefleau.banksystemdesktop.api.models.bankapi.response.get" to module com.google.gson
    at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:354)
Caused by: java.lang.reflect.InaccessibleObjectException: Unable to make field private final java.lang.String fr.plaglefleau.banksystemdesktop.api.models.bankapi.response.get.LoginResponse.message accessible: module fr.plaglefleau.banksystemdesktop does not "opens fr.plaglefleau.banksystemdesktop.api.models.bankapi.response.get" to module com.google.gson

    at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:297)
    at java.base/java.lang.reflect.Field.checkCanSetAccessible(Field.java:178)
    at java.base/java.lang.reflect.Field.setAccessible(Field.java:172)
    at com.google.gson@2.10.1/com.google.gson.internal.reflect.ReflectionHelper.makeAccessible(ReflectionHelper.java:35)
    ... 19 more

当前module-info.java代码：

module fr.plaglefleau.banksystemdesktop {
    requires javafx.controls;
    requires javafx.fxml;
    requires kotlin.stdlib;
    requires TranslationLibrary.main;
    requires kotlinx.coroutines.core;
    requires org.jetbrains.annotations;
    requires retrofit2;
    requires retrofit2.converter.gson;
    requires com.google.gson;

    opens fr.plaglefleau.banksystemdesktop to javafx.fxml, retrofit2.converter.gson;
    exports fr.plaglefleau.banksystemdesktop;
}

解决方案 #

错误根源是Java模块系统未向com.google.gson开放实体类所在的具体子包。你当前仅开放了根包fr.plaglefleau.banksystemdesktop，但Gson需要访问的是fr.plaglefleau.banksystemdesktop.api.models.bankapi.response.get下的LoginResponse类，必须明确开放该包。

修改module-info.java，添加对应包的开放声明：

module fr.plaglefleau.banksystemdesktop {
    requires javafx.controls;
    requires javafx.fxml;
    requires kotlin.stdlib;
    requires TranslationLibrary.main;
    requires kotlinx.coroutines.core;
    requires org.jetbrains.annotations;
    requires retrofit2;
    requires retrofit2.converter.gson;
    requires com.google.gson;

    opens fr.plaglefleau.banksystemdesktop to javafx.fxml, retrofit2.converter.gson;
    // 开放LoginResponse所在的包给Gson
    opens fr.plaglefleau.banksystemdesktop.api.models.bankapi.response.get to com.google.gson;
    // 若有其他实体类子包，同理添加开放声明
    // opens fr.plaglefleau.banksystemdesktop.api.models.bankapi.response.post to com.google.gson;
    exports fr.plaglefleau.banksystemdesktop;
}

如果所有实体类都集中在api.models父包下，也可以一次性开放整个父包，简化配置：

opens fr.plaglefleau.banksystemdesktop.api.models to com.google.gson;

这样Gson就能通过反射访问实体类的私有字段，完成JSON与实体对象的转换。

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