我有一个Item类（见下文），其中使用了一些Room db注释。它还有一个名为ItemInfo的嵌套类。这两个类都有一个空的构造函数。
问题是当我尝试序列化Item类的对象时，应用程序崩溃并出现以下错误：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.android.carrymates, PID: 18526
java.lang.SecurityException: Can not make a java.lang.reflect.Method constructor accessible
at java.lang.reflect.AccessibleObject.setAccessible0(AccessibleObject.java:133)
at java.lang.reflect.AccessibleObject.setAccessible(AccessibleObject.java:119)
at com.google.gson.internal.reflect.PreJava9ReflectionAccessor.makeAccessible(PreJava9ReflectionAccessor.java:31)
at com.google.gson.internal.ConstructorConstructor.newDefaultConstructor(ConstructorConstructor.java:103)
at com.google.gson.internal.ConstructorConstructor.get(ConstructorConstructor.java:85)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:101)
at com.google.gson.Gson.getAdapter(Gson.java:458)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.createBoundField(ReflectiveTypeAdapterFactory.java:117)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:166)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102)
at com.google.gson.Gson.getAdapter(Gson.java:458)
at com.google.gson.internal.bind.ArrayTypeAdapter$1.create(ArrayTypeAdapter.java:48)
at com.google.gson.Gson.getAdapter(Gson.java:458)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.createBoundField(ReflectiveTypeAdapterFactory.java:117)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:166)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102)
at com.google.gson.Gson.getAdapter(Gson.java:458)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.createBoundField(ReflectiveTypeAdapterFactory.java:117)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:166)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102)
at com.google.gson.Gson.getAdapter(Gson.java:458)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.createBoundField(ReflectiveTypeAdapterFactory.java:117)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:166)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102)
at com.google.gson.Gson.getAdapter(Gson.java:458)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.createBoundField(ReflectiveTypeAdapterFactory.java:117)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:166)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102)
at com.google.gson.Gson.getAdapter(Gson.java:458)
at com.google.gson.Gson.toJson(Gson.java:696)
at com.google.gson.Gson.toJson(Gson.java:683)
at com.google.gson.Gson.toJson(Gson.java:638)
at com.google.gson.Gson.toJson(Gson.java:618)
... more log (irrelevant to question asked)
Item.java
@Entity(tableName = "items", indices = {@Index(value = {"id"}, unique = true), @Index(value = {"owner", "type"})})
public class Item {
@PrimaryKey
@NonNull
String id="";
//...rest fields are int, boolean and String only
@Embedded
ItemInfo itemInfo; // see ItemInfo
public Item() {
}
// ...getters and setters
@IgnoreExtraProperties
public static class ItemInfo {
//...fields are int, boolean and String only
public ItemInfo() {
}
//...getters and setters
}
}
我的猜测是，Room DB批注将添加至少一个类型为java.lang.reflect.Method的对象，而Gson无法序列化该对象。
下面是我用来将Item对象序列化为json字符串的代码，其中item是Item类的对象，其字段类型为String和ItemInfo的字段为非空
Gson gson = new Gson();
String result = gson.toJson(item); // crash begins from here
我该如何解决这个问题？我希望至少有一种解决方法。
答案 0 :(得分：0)
我建议您将不同的对象用于不同的目标（存储在Room数据库中并序列化为json）。
您需要具有 Item 实体的界面：
public interface Item {
int getId();
//other fields
}
然后，您需要一个Room数据库实体的特定实现。您实际上已经拥有的东西，但是需要进行相同的重构：
@Entity(tableName = "items", indices = {@Index(value = {"id"}, unique = true), @Index(value = {"owner", "type"})})
public class RoomItem implements Item {
@PrimaryKey
@NonNull
private int id;
//other fields
public RoomItem() {
}
public RoomItem(Item item) {
id = item.getId();
}
@Override
public int getId() {
return 0;
}
@Override
public void setId(int id) {
this.id = id;
}
//other getters and setters
}
此外，您需要摆脱内部静态类 ItemInfo ，并将其放在单独的.java文件中。
最后，您需要针对Gson实体的特定实现：
public class GsonItem implements Item {
private final int id;
public GsonItem(Item origin) {
id = origin.getId();
}
@Override
public int getId() {
return id;
}
}
在这种情况下，您可以像这样使用它而不会出现任何问题：
Gson gson = new Gson();
String result = gson.toJson(new GsonItem(item));
是的，这种方法会使您编写更多代码，但是缺少像您这样的意外问题肯定会花费很多精力！
答案 1 :(得分：0)
请尝试以下操作：这段代码已正确插入
import android.arch.persistence.room.Embedded;
import android.arch.persistence.room.Entity;
import android.arch.persistence.room.Index;
import android.arch.persistence.room.PrimaryKey;
import android.support.annotation.NonNull;
import com.google.firebase.database.IgnoreExtraProperties;
@Entity(tableName = "items", indices = {@Index(value = {"id"}, unique = true), @Index(value = {"owner", "type"})})
public class Item {
@PrimaryKey
@NonNull
String id="";
//...rest fields are int, boolean and String only
@Embedded
ItemInfo itemInfo; // see ItemInfo
public Item() {
}
// ...getters and setters
@IgnoreExtraProperties
public static class ItemInfo {
//...fields are int, boolean and String only
public ItemInfo() {
}
int prop1;
String id="";
//...getters and setters
}
}
请注意也要删除依赖项
implementation 'com.google.firebase:firebase-core:16.0.3'
implementation "com.google.firebase:firebase-database:16.0.1"
// Arch
implementation "android.arch.core:runtime:1.1.1"
implementation "android.arch.core:common:1.1.1"
implementation 'android.arch.persistence.room:runtime:1.1.1';
annotationProcessor 'android.arch.persistence.room:compiler:1.1.1';
和实现：
Item item = new Item();
item.id = "Rover";
item.itemInfo = new Item.ItemInfo();
item.itemInfo.id = "asd";
item.itemInfo.prop1 = 1;
Gson gson = new Gson();
String json = gson.toJson(item); // here json ={"id":"Rover","itemInfo":{"id":"asd","prop1":1}}
答案 2 :(得分：0)
请勿嵌套这些类，以使用@Embedded批注：
@Entity(
tableName = "items",
indices = {
@Index(value = {"id"}, unique = true),
@Index(value = {"owner", "type"})
}
)
public class Item {
@PrimaryKey
@ColumnInfo(name = "id")
String id = null;
@Embedded
ItemInfo itemInfo;
public Item() {
}
}
@Entity(tableName = "item_info")
public class ItemInfo {
public ItemInfo() {
}
...
}
另请参阅此answer，涉及GSON的排除策略（itemInfo可能需要）
或直接将这些字段直接添加到类Item中，以便一次全部序列化它们-
为了不增加不必要的复杂性，只会造成问题。
答案 3 :(得分：0)
免责声明：
我不知道 RoomDB 对@Entity类有什么作用（尽管看起来RoomDB使用子类而不是您编写的类）。
我也在JVM上运行测试
但是我建议您使用@Expose：
public class GsonTest {
private static class SampleModel {
@Expose
private int i;
private Method method;
@Expose
private Nested nested = new Nested();
}
private static class Nested {
@Expose
private String a = "my string";
}
@Test
public void failsWithMethodField() throws Exception {
assertThrows(Exception.class, () -> {
SampleModel sampleModel = new SampleModel();
sampleModel.i = 10;
sampleModel.method = Object.class.getDeclaredMethod("equals", Object.class);
Gson gson = new Gson();
gson.toJson(sampleModel);
});
}
@Test
public void withExposedDoesNotFail() {
assertDoesNotThrow(() -> {
SampleModel sampleModel = new SampleModel();
sampleModel.method = Object.class.getDeclaredMethod("equals", Object.class);
Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create();
String json = gson.toJson(sampleModel);
System.out.println(json); // {"i":0,"nested":{"a":"my string"}}
});
}
}
必不可少的部分是使用Gson选项配置excludeFieldsWithoutExposeAnnotation：
Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create();
然后用@Expose注释标记序列化和反序列化时应使用的所有字段。