Java Record 的一些思考 - 序列化相关

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：

1. 自动生成的 private final field
2. 自动生成的全属性构造器
3. 自动生成的 public getter 方法
4. 自动生成的 hashCode()，equals()，toString() 方法：
5. 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
6. 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。

• 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

• 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

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：

1. 实现一个针对 Record 的专用的 Serializer 以及Deserializer。
2. 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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