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修复 kotlin data class 在字段超过 32 个的情况下默认值不生效的问题
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@getActivity
getActivity committed Dec 16, 2023
1 parent becfc7f commit 41a2397
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246 changes: 11 additions & 235 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -20,7 +20,7 @@ allprojects {
```groovy
dependencyResolutionManagement {
repositories {
// JitPack 远程仓库：https://jitpack.io[NameThatColor-1.7.4-fix.jar](..%2FStudioPlugins%2Fplugin%2FNameThatColor-1.7.4-fix.jar)
// JitPack 远程仓库：https://jitpack.io
maven { url 'https://jitpack.io' }
}
}
Expand All @@ -39,13 +39,19 @@ android {
dependencies {
// Gson 解析容错：https://github.com/getActivity/GsonFactory
implementation 'com.github.getActivity:GsonFactory:9.0'
implementation 'com.github.getActivity:GsonFactory:9.2'
// Json 解析框架：https://github.com/google/gson
implementation 'com.google.code.gson:gson:2.10.1'
// Kotlin 反射库：用于反射 Kotlin data class 类对象
implementation 'org.jetbrains.kotlin:kotlin-reflect:1.5.10'
}
```

* 需要注意的是：Gson 框架必须使用 **2.9.0** 及以上版本，否则将会出现版本兼容问题
* 需要注意两点：

* 当前项目必须要有 Kotlin 环境，否则会编译不通过

* Gson 框架必须使用 **2.9.0** 及以上版本，否则将会出现版本兼容问题

#### 使用文档

Expand Down Expand Up @@ -166,246 +172,16 @@ class XxxBean {
* 如果你在 Kotlin 中定义了以下内容的 Bean 类

```kotlin
data class DataClassBean(val name: String?, val age: Int = 18)
data class DataClassBean(val name: String = "Hello")
```

* 如果丢给 Gson 解析，最终会得到以下结果

```text
name = null
age = 0
```

* age 为什么不等于 18？为什么会等于 0 呢？要知道这个问题的原因，我们需要反编译看一下 DataClassBean 的源码

```java
public final class DataClassBean {
private final int age;
private final String name;

public static /* synthetic */ DataClassBean copy$default(DataClassBean bean, String str, int i, int i2, Object obj) {
if ((i2 & 1) != 0) {
str = bean.name;
}
if ((i2 & 2) != 0) {
i = bean.age;
}
return bean.copy(str, i);
}

public final String component1() {
return this.name;
}

public final int component2() {
return this.age;
}

public final DataClassBean copy(String str, int i) {
return new DataClassBean(str, i);
}

public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj instanceof DataClassBean) {
DataClassBean bean = (DataClassBean) obj;
return Intrinsics.areEqual(this.name, bean.name) && this.age == bean.age;
}
return false;
}

public int hashCode() {
String str = this.name;
return ((str == null ? 0 : str.hashCode()) * 31) + this.age;
}

public String toString() {
return "DataClassBean(name=" + ((Object) this.name) + ", age=" + this.age + ')';
}

public DataClassBean(String name, int age) {
this.name = name;
this.age = age;
}

public /* synthetic */ DataClassBean(String str, int i, int i2, DefaultConstructorMarker defaultConstructorMarker) {
this(str, (i2 & 2) != 0 ? 18 : i);
}

public final int getAge() {
return this.age;
}

public final String getName() {
return this.name;
}
}
```

* 不知道大家发现问题没有？DataClassBean 类里面并没有空参构造函数，那 Gson 到底是怎么创建对象的呢？让我们看一段源码

```java
package com.google.gson.internal;

public final class ConstructorConstructor {

public <T> ObjectConstructor<T> get(TypeToken<T> typeToken) {

......

ObjectConstructor<T> defaultConstructor = newDefaultConstructor(rawType, filterResult);
if (defaultConstructor != null) {
return defaultConstructor;
}

......

if (filterResult == FilterResult.ALLOW) {
// finally try unsafe
return newUnsafeAllocator(rawType);
} else {
........
}
}

private <T> ObjectConstructor<T> newUnsafeAllocator(final Class<? super T> rawType) {

......

ObjectConstructor<T> defaultConstructor = newDefaultConstructor(rawType, filterResult);
if (defaultConstructor != null) {
return defaultConstructor;
}

......

if (useJdkUnsafe) {
return new ObjectConstructor<T>() {
@Override public T construct() {
try {
@SuppressWarnings("unchecked")
T newInstance = (T) UnsafeAllocator.INSTANCE.newInstance(rawType);
return newInstance;
} catch (Exception e) {
throw new RuntimeException(("Unable to create instance of " + rawType + "."
+ " Registering an InstanceCreator or a TypeAdapter for this type, or adding a no-args"
+ " constructor may fix this problem."), e);
}
}
};
} else {
......
}
}

private static <T> ObjectConstructor<T> newDefaultConstructor(Class<? super T> rawType, FilterResult filterResult) {

......

final Constructor<? super T> constructor;
try {
constructor = rawType.getDeclaredConstructor();
} catch (NoSuchMethodException e) {
return null;
}

......

return new ObjectConstructor<T>() {
@Override public T construct() {
try {
@SuppressWarnings("unchecked") // T is the same raw type as is requested
T newInstance = (T) constructor.newInstance();
return newInstance;
}
// Note: InstantiationException should be impossible because check at start of method made sure
// that class is not abstract
catch (InstantiationException e) {
throw new RuntimeException("Failed to invoke constructor '" + ReflectionHelper.constructorToString(constructor) + "'"
+ " with no args", e);
} catch (InvocationTargetException e) {
// TODO: don't wrap if cause is unchecked?
// TODO: JsonParseException ?
throw new RuntimeException("Failed to invoke constructor '" + ReflectionHelper.constructorToString(constructor) + "'"
+ " with no args", e.getCause());
} catch (IllegalAccessException e) {
throw ReflectionHelper.createExceptionForUnexpectedIllegalAccess(e);
}
}
};
}
}
```

```java
package com.google.gson.internal;

public abstract class UnsafeAllocator {

public abstract <T> T newInstance(Class<T> c) throws Exception;

public static final UnsafeAllocator INSTANCE = create();

private static UnsafeAllocator create() {
// try JVM
// public class Unsafe {
// public Object allocateInstance(Class<?> type);
// }
try {
Class<?> unsafeClass = Class.forName("sun.misc.Unsafe");
Field f = unsafeClass.getDeclaredField("theUnsafe");
f.setAccessible(true);
final Object unsafe = f.get(null);
final Method allocateInstance = unsafeClass.getMethod("allocateInstance", Class.class);
return new UnsafeAllocator() {
@Override
@SuppressWarnings("unchecked")
public <T> T newInstance(Class<T> c) throws Exception {
assertInstantiable(c);
return (T) allocateInstance.invoke(unsafe, c);
}
};
} catch (Exception ignored) {
// OK: try the next way
}

......
}
}
```

* 相信你看完就懂了，Gson 确实是反射创建无参构造函数来创建对象，但是如果没有空参构造函数的情况下，它也会通过另外的手段创建对象，借助 `sun.misc.Unsafe` 创建对象，这样会有一个问题，这样创建出来的对象它不会走任何构造函数，通过查看刚刚反编译出来的 DataClassBean 类，就知道为什么这样 Kotlin 默认值都不会生效了

* 框架的做法很简单，既然没有无参构造函数，那我就通过其他构造函数来创建，就拿 Kotlin 生成的 `DataClassBean(String str, int i, int i2, DefaultConstructorMarker defaultConstructorMarker)` 来创建对象

这个构造函数特别有意思，最后第一个参数是 DefaultConstructorMarker 类，里面啥也没有

```java
public final class DefaultConstructorMarker {
private DefaultConstructorMarker() {
}
}
```

* 最后第二个参数是参数标记，标记是否使用 data class 定义的默认值

```java
public final class DataClassBean {

public /* synthetic */ DataClassBean(String str, int i, int i2, DefaultConstructorMarker defaultConstructorMarker) {
this(str, (i2 & 2) != 0 ? 18 : i);
}

public DataClassBean(String name, int age) {
this.name = name;
this.age = age;
}
}
```

* 框架的解决方案是：反射最后第一个参数类型为 DefaultConstructorMarker，然后传入空对象即可，最后第二个参数类型为 int 的构造函数，并且让最后第二个参数的位运算逻辑为 true，让它走到默认值赋值那里，这样可以选择传入 `Integer.MAX_VALUE`，这样每次使用它去 & 不大于 0 的某个值，都会等于某个值，也就是不会等于 0，这样就能保证它的运算条件一直为 true，也就是使用默认值，其他参数传值的话，如果是基本数据类型，就传入基本数据类型的默认值，如果是对象类型，则直接传入 null。这样就解决了 Gson 反射 Kotlin Data Class 类出现字段默认值不生效的问题。
* `name` 字段为什么不等于 `Hello` ？为什么会等于 `null` 值呢？这是因为 Gson 默认只初始化了 DataClassBean 类的空参构造函数，框架的解决方案很简单粗暴，直接引入 Kotlin 反射库，让它找到 Kotlin data class 自动生成的主构造函数，然后反射创建 Kotlin 类，这样得到的对象，非空字段的默认值都会被保留，这样就解决了 Gson 反射 Kotlin Data Class 类出现字段默认值不生效的问题，目前框架内部已经处理了该问题，外部使用的人无需额外处理该问题，只需要调用框架进行解析即可。

## 常见疑问解答

Expand Down
4 changes: 2 additions & 2 deletions app/build.gradle
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Expand Up @@ -8,8 +8,8 @@ android {
applicationId "com.hjq.gson.factory.demo"
minSdkVersion 16
targetSdkVersion 31
versionCode 900
versionName "9.0"
versionCode 920
versionName "9.2"
testInstrumentationRunner 'androidx.test.runner.AndroidJUnitRunner'
}

Expand Down
2 changes: 1 addition & 1 deletion build.gradle
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Expand Up @@ -16,7 +16,7 @@ buildscript {
classpath 'com.android.tools.build:gradle:4.1.2'
// Kotlin 插件：https://plugins.jetbrains.com/plugin/6954-kotlin
// noinspection GradleDependency
classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.5.31'
classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.5.32'
}
}

Expand Down
9 changes: 6 additions & 3 deletions library/build.gradle
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@@ -1,12 +1,13 @@
apply plugin: 'com.android.library'
apply plugin: 'kotlin-android'

android {
compileSdkVersion 31

defaultConfig {
minSdkVersion 11
versionCode 900
versionName "9.0"
minSdkVersion 12
versionCode 920
versionName "9.2"
}

// 使用 JDK 1.8
Expand Down Expand Up @@ -35,6 +36,8 @@ dependencies {
// Json 解析框架：https://github.com/google/gson
// noinspection GradleDependency
implementation 'com.google.code.gson:gson:2.10.1'
// Kotlin 反射依赖：用于反射 Kotlin data class 类对象
implementation 'org.jetbrains.kotlin:kotlin-reflect:1.5.10'
}

// 防止编码问题
Expand Down
4 changes: 2 additions & 2 deletions library/src/main/java/com/hjq/gson/factory/ParseExceptionCallback.java
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Expand Up @@ -25,7 +25,7 @@ public interface ParseExceptionCallback {
*
* @param typeToken 类型 Token
* @param fieldName 字段名称（可能为空）
* @param listItemJsonToken List 条目类型
* @param listItemJsonToken List 条目类型（可能为空）
*/
void onParseListException(TypeToken<?> typeToken, String fieldName, JsonToken listItemJsonToken);

Expand All @@ -35,7 +35,7 @@ public interface ParseExceptionCallback {
* @param typeToken 类型 Token
* @param fieldName 字段名称（可能为空）
* @param mapItemKey Map 集合中的 key 值，如果等于为 "null" 字符串，则证明后端返回了错误类型的 key 过来
* @param mapItemJsonToken Map 条目类型
* @param mapItemJsonToken Map 条目类型（可能为空）
*/
void onParseMapException(TypeToken<?> typeToken, String fieldName, String mapItemKey, JsonToken mapItemJsonToken);
}
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