我在 Java 类中有一个 final 字段，它肯定会被初始化。当我序列化和反序列化相应的对象图时，我得到一个 NPE，因为该字段在 hashCode() 方法中使用但显然尚未回读。我创建了一个最小的测试用例，其中还包含 serialize()/deserialize() 方法(以防这些有问题)，但我似乎无法了解(潜在的)问题是什么或如何解决。

public class TestSerializerTest {

private static final org.slf4j.Logger logger = org.slf4j.LoggerFactory.getLogger(TestSerializerTest.class);

private File workDir;

@Before
public void setUp() {
    workDir = new File("target/tmp");
}

@After
public void tearDown() {
    workDir.delete();
}

private static class ManagingContainer implements Serializable {

    private static final long serialVersionUID = 1L;

    private final Set<Contained> containedElements = new HashSet<Contained>();

    public Contained getContained(List<String> descriptors) {
        Contained contained = new Contained(this, descriptors);
        containedElements.add(contained);
        return contained;
    }
}

private static class Contained implements Serializable {

    private static final long serialVersionUID = 1L;

    @SuppressWarnings("unused")
    private final ManagingContainer container;
    private final List<String> descriptors;

    Contained(ManagingContainer container, List<String> descriptors) {
        this.container = container;
        if (descriptors == null) {
            throw new NullPointerException();
        }
        this.descriptors = new ArrayList<String>(descriptors);
    }

    @Override
    public int hashCode() {
        return descriptors.hashCode();
    }

    @Override
    public boolean equals(Object other) {
        if (this == other) {
            return true;
        }
        if (other == null || !(other instanceof Contained)) {
            return false;
        }
        return this.descriptors.equals(((Contained) other).descriptors);
    }
}

private static class OtherContainer implements Serializable {
    private static final long serialVersionUID = 1L;

    private final ArrayList<Contained> containedElements = new ArrayList<Contained>();

    public OtherContainer(Contained initialElement) {
        this.containedElements.add(initialElement);
    }

    public void addContained(Contained nextElement) {
        containedElements.add(nextElement);
    }
}

void serializeObjectToFile(Serializable serializable, File file) {
    logger.info("Saving object '{}' to file '{}'.", serializable, file.getAbsolutePath());
    ObjectOutputStream stream = null;
    try {
        if (!file.getParentFile().exists()) {
            logger.info("Creating directory '{}'.", file.getParentFile().getAbsolutePath());
            file.getParentFile().mkdirs();
        }
        stream = new ObjectOutputStream(new FileOutputStream(file));
        stream.writeObject(serializable);
        stream.flush();
    } catch (Exception exception) {
        throw new RuntimeException(exception);
    } finally {
        if (stream != null) {
            try {
                stream.close();
            } catch (IOException exc) {
                logger.error("Error closing stream.", exc);
            }
        }
    }
}

Object deserializeObjectFromFile(File file) {
    ObjectInputStream oiStream = null;
    try {
        logger.info("Loading object from file '{}'.", file.getAbsolutePath());
        oiStream = new ObjectInputStream(new FileInputStream(file));
        return oiStream.readObject();
    } catch (Exception exc) {
        logger.error("Exception loading object from file '{}'. Ignoring file!", file.getAbsolutePath(), exc);
        throw new RuntimeException(exc);
    } finally {
        if (oiStream != null) {
            try {
                oiStream.close();
            } catch (IOException exc) {
                logger.error("Error closing stream.", exc);
            }
        }
    }
}

@Test
public void testSerializeDeserialize() {
    ManagingContainer container = new ManagingContainer();
    OtherContainer serializable = new OtherContainer(container.getContained(new ArrayList<String>()));
    serializable.addContained(container.getContained(new ArrayList<String>()));
    File file = new File(workDir, "test.ser");
    serializeObjectToFile(serializable, file);
    OtherContainer result = (OtherContainer) deserializeObjectFromFile(file);
}
}

执行此测试会产生以下 NullPointerException:

java.lang.NullPointerException: null
at TestSerializerTest$Contained.hashCode(TestSerializerTest.java:67) ~[test-classes/:na]
at java.util.HashMap.put(HashMap.java:372) ~[na:1.6.0_29]
at java.util.HashSet.readObject(HashSet.java:292) ~[na:1.6.0_29]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.6.0_29]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) ~[na:1.6.0_29]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) ~[na:1.6.0_29]
at java.lang.reflect.Method.invoke(Method.java:597) ~[na:1.6.0_29]
at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:974) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1848) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328) ~[na:1.6.0_29]
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328) ~[na:1.6.0_29]
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350) ~[na:1.6.0_29]
at java.util.LinkedList.readObject(LinkedList.java:964) ~[na:1.6.0_29]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.6.0_29]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) ~[na:1.6.0_29]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) ~[na:1.6.0_29]
at java.lang.reflect.Method.invoke(Method.java:597) ~[na:1.6.0_29]
at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:974) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1848) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328) ~[na:1.6.0_29]
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1946) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1870) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328) ~[na:1.6.0_29]
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350) ~[na:1.6.0_29]
at TestSerializerTest.deserializeObjectFromFile(TestSerializerTest.java:128) [test-classes/:na]
at TestSerializerTest.testSerializeDeserialize(TestSerializerTest.java:151) [test-classes/:na]

据我了解序列化，这应该不是问题(另请参阅 Does Java Serialization work for cyclic references? 和 https://softwareengineering.stackexchange.com/questions/151055/what-happens-if-we-serialize-and-deserialize-two-objects-which-references-to-eac)。

目前我不知道如何进行。任何提示都非常感谢!谢谢!

最佳答案

问题好像是ManagingContainer和Container之间的循环关系引起的。当 Container 被反序列化时，它反过来反序列化“container”属性引用的 ManagingContainer。但是，在反序列化过程中，它会尝试使用正在反序列化过程中的容器填充 HashSet。

如果您序列化/反序列化而不是您的 ManagingContainer 它应该可以正常工作，因为在调用 hashCode() 之前容器将被完全加载。或者，重新考虑您的对象图以删除循环依赖或编写自定义对象读/写方法。

关于java - 反序列化对象图时 hashCode() 中的 NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/26971127/