Java Record 的一些思考 – 序列化相关
- 2022 年 1 月 4 日
- 筆記
- JAVA, Java 展望, 实战 Java 16 值类型 Record
Java Record 序列化相关
Record 在设计之初，就是为了找寻一种纯表示数据的类型载体。Java 的 class 现在经过不断的迭代做功能加法，用法已经非常复杂，各种语法糖，各种多态构造器，各种继承设计导致针对 Java 的序列化框架也做得非常复杂，要考虑的情况有很多很多。每次 Java 升级，如果对类结构有做改动或者加入了新特性，那么序列化框架就都需要改来兼容。这样会阻碍 Java 的发展，于是设计出了 Record 这个专门用来存储数据的类型。
经过上一节的分析我们知道，Record 类型声明后就是 final 的，在编译后，根据 Record 源码插入相关域与方法的字节码，包括：
- 自动生成的 private final field
- 自动生成的全属性构造器
- 自动生成的 public getter 方法
- 自动生成的 hashCode()，equals()，toString() 方法：
- 从字节码可以看出，这三个方法的底层实现是 invokeDynamic 另一个方法
- 调用的是
ObjectMethods.java这个类中的bootstrap方法
里面的所有元素都是不可变的，这样对序列化来讲方便了很多，省略掉很多要考虑的因素，比如字段父子类继承与覆盖等等。序列化一个 Record，只需要关注这个 Record 本身，将其中的所有 field 读取出来即可，并且这些 field 都是 final 的。反序列化的时候，仅通过 Record 的规范构造函数（canonical constructor）即给全属性赋值的构造函数。
接下来我们通过一个简单的例子来看下 Record 与普通类的序列化区别。
我们在这里使用了 lombok 简化代码，假设有 UserClass：
@Data
public class UserClass implements Serializable {
private final int id;
private final int age;
}
还有与它有相同 field 的 UserRecord：
public record UserRecord(int id, int age) implements Serializable {}
编写使用 Java 原生序列化的代码：
public class SerializationTest {
public static void main(String[] args) throws Exception {
try (FileOutputStream fileOutputStream = new FileOutputStream("data");
ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream)) {
//先写入 UserClass
objectOutputStream.writeObject(new UserClass(1, -1));
//再写入 UserRecord
objectOutputStream.writeObject(new UserRecord(2, -1));
}
}
}
执行，将两个对象写入了文件 data 中，然后，再编写代码从这个文件中读取出来并输出：
public class DeSerializationTest {
public static void main(String[] args) throws Exception {
try (FileInputStream fileInputStream = new FileInputStream("data");
ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream)) {
//读取 UserClass
System.out.println(objectInputStream.readObject());
//读取 UserRecord
System.out.println(objectInputStream.readObject());
}
}
}
执行后，会看到输出：
UserClass(id=1, age=-1)
UserRecord[id=1, age=-1]
构造器测试
接下来，我们修改下源码，在 UserClass 和 UserRecord 中增加 id 和 age 都不能小于 1 的判断。并且，额外给 UserRecord 增加一个构造器，来验证反序列化使用的是 UserRecord 全属性构造器。
@Data
public class UserClass implements Serializable {
private final int id;
private final int age;
public UserClass(int id, int age) {
if (id < 0 || age < 0) {
throw new IllegalArgumentException("id and age should be larger than 0");
}
this.id = id;
this.age = age;
}
}
public record UserRecord(int id, int age) implements Serializable {
public UserRecord {
if (id < 0 || age < 0) {
throw new IllegalArgumentException("id and age should be larger than 0");
}
}
public UserRecord(int id) {
this(id, 0);
}
}
再次执行代码 DeSerializationTest，我们会发现有报错，但是 UserClass 被反序列化出来了：
UserClass(id=1, age=-1)
Exception in thread "main" java.io.InvalidObjectException: id and age should be larger than 0
at java.base/java.io.ObjectInputStream.readRecord(ObjectInputStream.java:2348)
at java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2236)
at java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1742)
at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:514)
at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:472)
at DeSerializationTest.main(DeSerializationTest.java:13)
Caused by: java.lang.IllegalArgumentException: id and age should be larger than 0
at UserRecord.<init>(UserRecord.java:6)
at java.base/java.io.ObjectInputStream.readRecord(ObjectInputStream.java:2346)
... 5 more
兼容性测试
我们再来看如果删除一个字段会怎么样：
@Data
public class UserClass implements Serializable {
private final int age;
}
public record UserRecord(int age) implements Serializable {
}
执行代码，读取 UserClass 的时候就会报错，这也是符合预期的，因为这在普通类对象的反序列化说明中就说这种是不兼容修改。将 UserClass 的字段恢复，重新执行代码，发现成功：
UserClass(id=1, age=-1)
UserRecord[age=-1]
也就是说，Record 是默认兼容缺失字段的反序列化的
我们将字段恢复，再来看多一个字段会怎么样：
@Data
public class UserClass implements Serializable {
private final int id;
private final int sex;
private final int age;
}
public record UserRecord(int id, int sex, int age) implements Serializable {
}
执行代码，读取 UserClass 的时候就会报错，这也是符合预期的。将 UserClass 的字段恢复，重新执行代码，发现成功：
UserClass(id=1, age=-1)
UserRecord[id=2, sex=0, age=-1]
也就是说，Record 是默认兼容字段变多的反序列化的
最后测试一下 Record 的 field 类型如果变了呢：
public record UserRecord(int id, Integer age) implements Serializable {
}
执行代码发现失败，因为类型不匹配了(就算是包装类也不行)：
UserClass(id=1, age=-1)
Exception in thread "main" java.io.InvalidClassException: UserRecord; incompatible types for field age
at java.base/java.io.ObjectStreamClass.matchFields(ObjectStreamClass.java:2391)
at java.base/java.io.ObjectStreamClass.getReflector(ObjectStreamClass.java:2286)
at java.base/java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:788)
at java.base/java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:2060)
at java.base/java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1907)
at java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2209)
at java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1742)
at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:514)
at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:472)
at DeSerializationTest.main(DeSerializationTest.java:13)
一些主流的序列化框架的兼容
由于 Record 限制了序列化与反序列化的唯一方式，所以其实兼容起来很简单，比起 Java Class 改个结构，加个特性导致的序列化框架更改来说还要简单。
这三个框架中实现对于 Record 的兼容思路都很类似，也比较简单，即：
- 实现一个针对 Record 的专用的 Serializer 以及Deserializer。
- 通过反射（Java Reflection）或者句柄（Java MethodHandle）验证当前版本的 Java 是否支持 Record，以及获取 Record 的规范构造函数（canonical constructor）以及各种 field 的 getter 进行反序列化和序列化。给大家两个工具类进行参考，分别是使用反射（Java Reflection）和句柄（Java MethodHandle）实现：
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.Comparator;
import common.RecComponent;
/**
* Utility methods for record serialization, using Java Core Reflection.
*/
public class ReflectUtils {
private static final Method IS_RECORD;
private static final Method GET_RECORD_COMPONENTS;
private static final Method GET_NAME;
private static final Method GET_TYPE;
static {
Method isRecord;
Method getRecordComponents;
Method getName;
Method getType;
try {
// reflective machinery required to access the record components
// without a static dependency on Java SE 14 APIs
Class<?> c = Class.forName("java.lang.reflect.RecordComponent");
isRecord = Class.class.getDeclaredMethod("isRecord");
getRecordComponents = Class.class.getMethod("getRecordComponents");
getName = c.getMethod("getName");
getType = c.getMethod("getType");
} catch (ClassNotFoundException | NoSuchMethodException e) {
// pre-Java-14
isRecord = null;
getRecordComponents = null;
getName = null;
getType = null;
}
IS_RECORD = isRecord;
GET_RECORD_COMPONENTS = getRecordComponents;
GET_NAME = getName;
GET_TYPE = getType;
}
/** Returns true if, and only if, the given class is a record class. */
static boolean isRecord(Class<?> type) {
try {
return (boolean) IS_RECORD.invoke(type);
} catch (Throwable t) {
throw new RuntimeException("Could not determine type (" + type + ")");
}
}
/**
* Returns an ordered array of the record components for the given record
* class. The order is imposed by the given comparator. If the given
* comparator is null, the order is that of the record components in the
* record attribute of the class file.
*/
static <T> RecComponent[] recordComponents(Class<T> type,
Comparator<RecComponent> comparator) {
try {
Object[] rawComponents = (Object[]) GET_RECORD_COMPONENTS.invoke(type);
RecComponent[] recordComponents = new RecComponent[rawComponents.length];
for (int i = 0; i < rawComponents.length; i++) {
final Object comp = rawComponents[i];
recordComponents[i] = new RecComponent(
(String) GET_NAME.invoke(comp),
(Class<?>) GET_TYPE.invoke(comp), i);
}
if (comparator != null) Arrays.sort(recordComponents, comparator);
return recordComponents;
} catch (Throwable t) {
throw new RuntimeException("Could not retrieve record components (" + type.getName() + ")");
}
}
/** Retrieves the value of the record component for the given record object. */
static Object componentValue(Object recordObject,
RecComponent recordComponent) {
try {
Method get = recordObject.getClass().getDeclaredMethod(recordComponent.name());
return get.invoke(recordObject);
} catch (Throwable t) {
throw new RuntimeException("Could not retrieve record components ("
+ recordObject.getClass().getName() + ")");
}
}
/**
* Invokes the canonical constructor of a record class with the
* given argument values.
*/
static <T> T invokeCanonicalConstructor(Class<T> recordType,
RecComponent[] recordComponents,
Object[] args) {
try {
Class<?>[] paramTypes = Arrays.stream(recordComponents)
.map(RecComponent::type)
.toArray(Class<?>[]::new);
Constructor<T> canonicalConstructor = recordType.getConstructor(paramTypes);
return canonicalConstructor.newInstance(args);
} catch (Throwable t) {
throw new RuntimeException("Could not construct type (" + recordType.getName() + ")");
}
}
}
package invoke;
import common.RecComponent;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.reflect.Array;
import java.util.Arrays;
import java.util.Comparator;
import static java.lang.invoke.MethodType.methodType;
/**
* Utility methods for record serialization, using MethodHandles.
*/
public class InvokeUtils {
private static final MethodHandle MH_IS_RECORD;
private static final MethodHandle MH_GET_RECORD_COMPONENTS;
private static final MethodHandle MH_GET_NAME;
private static final MethodHandle MH_GET_TYPE;
private static final MethodHandles.Lookup LOOKUP;
static {
MethodHandle MH_isRecord;
MethodHandle MH_getRecordComponents;
MethodHandle MH_getName;
MethodHandle MH_getType;
LOOKUP = MethodHandles.lookup();
try {
// reflective machinery required to access the record components
// without a static dependency on Java SE 14 APIs
Class<?> c = Class.forName("java.lang.reflect.RecordComponent");
MH_isRecord = LOOKUP.findVirtual(Class.class, "isRecord", methodType(boolean.class));
MH_getRecordComponents = LOOKUP.findVirtual(Class.class, "getRecordComponents",
methodType(Array.newInstance(c, 0).getClass()))
.asType(methodType(Object[].class, Class.class));
MH_getName = LOOKUP.findVirtual(c, "getName", methodType(String.class))
.asType(methodType(String.class, Object.class));
MH_getType = LOOKUP.findVirtual(c, "getType", methodType(Class.class))
.asType(methodType(Class.class, Object.class));
} catch (ClassNotFoundException | NoSuchMethodException e) {
// pre-Java-14
MH_isRecord = null;
MH_getRecordComponents = null;
MH_getName = null;
MH_getType = null;
} catch (IllegalAccessException unexpected) {
throw new AssertionError(unexpected);
}
MH_IS_RECORD = MH_isRecord;
MH_GET_RECORD_COMPONENTS = MH_getRecordComponents;
MH_GET_NAME = MH_getName;
MH_GET_TYPE = MH_getType;
}
/** Returns true if, and only if, the given class is a record class. */
static boolean isRecord(Class<?> type) {
try {
return (boolean) MH_IS_RECORD.invokeExact(type);
} catch (Throwable t) {
throw new RuntimeException("Could not determine type (" + type + ")");
}
}
/**
* Returns an ordered array of the record components for the given record
* class. The order is imposed by the given comparator. If the given
* comparator is null, the order is that of the record components in the
* record attribute of the class file.
*/
static <T> RecComponent[] recordComponents(Class<T> type,
Comparator<RecComponent> comparator) {
try {
Object[] rawComponents = (Object[]) MH_GET_RECORD_COMPONENTS.invokeExact(type);
RecComponent[] recordComponents = new RecComponent[rawComponents.length];
for (int i = 0; i < rawComponents.length; i++) {
final Object comp = rawComponents[i];
recordComponents[i] = new RecComponent(
(String) MH_GET_NAME.invokeExact(comp),
(Class<?>) MH_GET_TYPE.invokeExact(comp), i);
}
if (comparator != null) Arrays.sort(recordComponents, comparator);
return recordComponents;
} catch (Throwable t) {
throw new RuntimeException("Could not retrieve record components (" + type.getName() + ")");
}
}
/** Retrieves the value of the record component for the given record object. */
static Object componentValue(Object recordObject,
RecComponent recordComponent) {
try {
MethodHandle MH_get = LOOKUP.findVirtual(recordObject.getClass(),
recordComponent.name(),
methodType(recordComponent.type()));
return (Object) MH_get.invoke(recordObject);
} catch (Throwable t) {
throw new RuntimeException("Could not retrieve record components ("
+ recordObject.getClass().getName() + ")");
}
}
/**
* Invokes the canonical constructor of a record class with the
* given argument values.
*/
static <T> T invokeCanonicalConstructor(Class<T> recordType,
RecComponent[] recordComponents,
Object[] args) {
try {
Class<?>[] paramTypes = Arrays.stream(recordComponents)
.map(RecComponent::type)
.toArray(Class<?>[]::new);
MethodHandle MH_canonicalConstructor =
LOOKUP.findConstructor(recordType, methodType(void.class, paramTypes))
.asType(methodType(Object.class, paramTypes));
return (T)MH_canonicalConstructor.invokeWithArguments(args);
} catch (Throwable t) {
throw new RuntimeException("Could not construct type (" + recordType.getName() + ")");
}
}
}
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