我有一个 final 字段在一个Java类中肯定会被初始化。当我序列化和反序列化相应的对象图时，我得到了一个NPE，因为该字段是在 hashCode() 方法，但显然还没有读回来。我创建了一个最小测试用例，其中也包含了 serialize()deserialize() 方法（以防这些方法有问题），但我似乎无法理解（潜在的）问题是什么，或者如何解决它。
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);
}
}
执行这个测试会产生以下结果 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]
根据我的理解，这应该不是一个问题（另见 Java序列化对循环引用有用吗？ 和 https:/softwareengineering.stackexchange.comquestions151055what-happens-if-we-serialize-and-deserialize-two-objects-which-references-to-eac。).
目前我不知道该如何进行。任何提示都非常感谢! 谢谢！我在一个Java类中有一个最后的字段，这个字段肯定会被初始化。
这个问题似乎是由ManagingContainer和Container之间的循环关系引起的。当Container被反序列化时，它又反序列化了'container'属性所引用的ManagingContainer。然而，当这个反序列化时，它试图用正在被反序列化的Container填充HashSet。
如果你用serializedeserialize代替你的ManagingContainer，它应该可以正常工作，因为Containers将在hashCode()被调用之前被完全加载。另外，重新思考你的对象图，以消除循环依赖，或者编写自定义对象读写方法。
我在一个复杂的对象图中遇到了这个问题，该对象图使用HashMaps进行了双向循环引用。由于在调用hashCode()时，HashMap的key还没有完全加载，所以我也得到了一个NullPointerException。
private String id;
int hashCode() {
return id.hashCode(); // NPE during deserialization
}
我 "修复"（读作："避免"）了这个问题，同时对hashCode int本身进行了缓存和序列化。
private String id;
private int hashCode;
public void setId( String id ) {
this.id = id;
this.hashCode = id.hashCode();
}
public int hashCode() {
return this.hashCode;
}
基元在序列化过程中是内联的。它们会在任何对象引用（如上面的 id String）之前立即被加载。因此，即使没有加载任何被引用的 Objects，hashCode int 也是可用的。
坏消息是，这确实意味着只要提供hashCode的Object就需要更新hashCode。对于具有明确定义的访问update的对象，例如不可变的Strings，这是很直接的（参见上面的setId(...)）。对于具有复杂交互的对象，比如Collections，这就很难控制了，因为它可能取决于Collections内容的变化，也可能取决于Collections中各个对象的hashCodes。
好消息是，只要内存便宜，如果重复调用hashCode()，这可能会导致性能略有提高。