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
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 = newFileOutputStream("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 = newFileInputStream("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.(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。
- Jackson:
- Issue: Support for record types in JDK 14
- Pull Request: Support for record types in JDK 14
- 对应版本:jackson-databind-2.12.0
- 【Java Record 的一些思考 - 序列化相关】Kryo
- Issue: Java 14 records : how to deal with them?
- Pull Request: Add support for Records in JDK 14
- 对应版本:kryo-5.1.0
- XStream
- Issue: Support for record types in JDK 14
- Pull Request: Add support for Record types in JDK 14
- 对应版本:1.5.x,还未发布
- 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
RecComponent[] recordComponents(Class type, Comparator 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 invokeCanonicalConstructor(Class recordType, RecComponent[] recordComponents, Object[] args) { try { Class>[] paramTypes = Arrays.stream(recordComponents) .map(RecComponent::type) .toArray(Class>[]::new); Constructor 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 RecComponent[] recordComponents(Class type,
Comparator 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 invokeCanonicalConstructor(Class 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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