java - 为什么我在使用 jackson 时会收到 stackoverflow 错误，即使使用 @JsonIgnoreProperties

Question

我正在尝试使用 jackson 将 DefaultMutableTreeNode 对象序列化为 json 字符串。因此，我需要使用一个混合抽象类，它是 DefaultMutableTreeNode 类的代理。这可能是因为 self 引用字段，但我无法识别它们。

混合类:

@JsonIgnoreProperties(ignoreUnknown = true)
public abstract class DefaultMutableTreeNodeMixIn {

    @JsonCreator
    public DefaultMutableTreeNodeMixIn(@JsonProperty Object userObject) {};

    @JsonCreator
    public DefaultMutableTreeNodeMixIn(@JsonProperty Object userObject, 
    @JsonProperty boolean allowsChildren) {};

    @JsonProperty("childCount")
    abstract int getChildCount();

    @JsonProperty("depth")
    abstract int getDepth();

    @JsonProperty("firstChild")
    abstract TreeNode getFirstChild();

    @JsonProperty("firstLeaf")
    abstract DefaultMutableTreeNode getFirstLeaf();

    @JsonProperty("lastChild")
    abstract TreeNode getLastChild();

    @JsonProperty("lastLeaf")
    abstract DefaultMutableTreeNode getLastLeaf();

    @JsonProperty("leafCount")
    abstract int getLeafCount();

    @JsonProperty("level")
    abstract int getLevel();

    @JsonProperty("nextLeaf")
    abstract DefaultMutableTreeNode getNextLeaf();

    @JsonProperty("nextNode")
    abstract DefaultMutableTreeNode getNextNode();

    @JsonProperty("nextSibling")
    abstract DefaultMutableTreeNode getNextSibling();

    @JsonProperty("parent")
    abstract TreeNode getParent();

    @JsonProperty("path")
    abstract TreeNode[] getPath();

    @JsonProperty("previousLeaf")
    abstract DefaultMutableTreeNode getPreviousLeaf();

    @JsonProperty("previousNode")
    abstract DefaultMutableTreeNode getPreviousNode();

    @JsonProperty("previousSibling")
    abstract DefaultMutableTreeNode getPreviousSibling();

    @JsonProperty("siblingCount")
    abstract int getSiblingCount();

    @JsonProperty("isLeaf")
    abstract boolean isLeaf();

    @JsonProperty("isRoot")
    abstract boolean isRoot();
}

对象映射器:

ObjectMapper mapper = new ObjectMapper();
mapper.addMixIn(DefaultMutableTreeNode.class,DefaultMutableTreeNodeMixIn.class);
String json = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(serverFileTree);
System.out.println(json);

(serverFileTree是DefaultMutableTreeNode类型的对象)

错误跟踪:

at com.fasterxml.jackson.databind.ser.std.ObjectArraySerializer.serializeContents(ObjectArraySerializer.java:252)
at com.fasterxml.jackson.databind.ser.std.ObjectArraySerializer.serialize(ObjectArraySerializer.java:213)
at com.fasterxml.jackson.databind.ser.std.ObjectArraySerializer.serialize(ObjectArraySerializer.java:22)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155) [...]
Caused by: java.lang.StackOverflowError
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:737)
... 1011 more

Answer 1

当您开始遍历它们的 getter 方法时，此类会生成循环。要打破它们，您需要使用 JsonBackReference 注释。您的 mixin 可能如下所示:

@JsonIgnoreProperties(ignoreUnknown = true)
abstract class DefaultMutableTreeNodeMixIn {

    @JsonCreator
    public DefaultMutableTreeNodeMixIn(@JsonProperty Object userObject) {
    }

    @JsonCreator
    public DefaultMutableTreeNodeMixIn(@JsonProperty Object userObject, @JsonProperty boolean allowsChildren) {
    }

    @JsonProperty("childCount")
    abstract int getChildCount();

    @JsonProperty("depth")
    abstract int getDepth();

    @JsonProperty("firstChild")
    @JsonBackReference
    abstract TreeNode getFirstChild();

    @JsonProperty("firstLeaf")
    @JsonBackReference
    abstract DefaultMutableTreeNode getFirstLeaf();

    @JsonProperty("lastChild")
    @JsonBackReference
    abstract TreeNode getLastChild();

    @JsonProperty("lastLeaf")
    @JsonBackReference
    abstract DefaultMutableTreeNode getLastLeaf();

    @JsonProperty("leafCount")
    abstract int getLeafCount();

    @JsonProperty("level")
    abstract int getLevel();

    @JsonProperty("nextLeaf")
    abstract DefaultMutableTreeNode getNextLeaf();

    @JsonProperty("nextNode")
    abstract DefaultMutableTreeNode getNextNode();

    @JsonProperty("nextSibling")
    abstract DefaultMutableTreeNode getNextSibling();

    @JsonProperty("parent")
    abstract TreeNode getParent();

    @JsonProperty("path")
    @JsonBackReference
    abstract TreeNode[] getPath();

    @JsonProperty("previousLeaf")
    abstract DefaultMutableTreeNode getPreviousLeaf();

    @JsonProperty("previousNode")
    abstract DefaultMutableTreeNode getPreviousNode();

    @JsonProperty("previousSibling")
    abstract DefaultMutableTreeNode getPreviousSibling();

    @JsonProperty("siblingCount")
    abstract int getSiblingCount();

    @JsonProperty("isLeaf")
    abstract boolean isLeaf();

    @JsonProperty("isRoot")
    abstract boolean isRoot();
}

但最好和最OOP的方法可能是创建新的POJO，它代表你的树准备好序列化并且没有循环。