正如@Zutty所建议的：
在Jackson中新增了ObjectMapper（）.writeValueAsString（对象）
我试过了，但是得到一个com.fasterxml.Jackson.databind.jsonMappingException

com.fasterxml.jackson.databind.JsonMappingException: Direct self-reference leading to cycle (through reference chain: com.learnkeeper.shared.entities.RootBook["permissionRef"]->com.googlecode.objectify.impl.ref.LiveRef["key"]->com.googlecode.objectify.Key["root"])
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter._handleSelfReference(BeanPropertyWriter.java:781)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:569)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:666)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:156)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:575)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:666)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:156)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:575)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:666)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:156)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:129)
at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3385)
at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:2779)

当使用new Gson（）.toJson（Object）时，我会得到一个很好的Json：

{"id":4512395720392704,"origin":"FRENCH","target":"ENGLISH","wordKeys":[],"name":"Test","creation":"Aug 11, 2015 8:43:02 PM","modification":"Aug 11, 2015 8:43:02 PM","permissionRef":{"key":{"raw":{"kind":"Permission","id":6737807255011328}}},"lastVersion":true}

其他信息：我希望记录所有Java对象都实现了java.io.Serializable

takipi的工作人员对几个json解析器进行了comparison性能测试，包括gson和Jackson，结果发现这取决于被解析的文档类型，Gson处理小文档的速度更快，jackson处理大文档的速度更快，第三种选择json.simple在大文档和小文档上都提供了更一致的性能。

还有几个不同之处需要补充：

• Gson使用字段值填充JSON字符串，而Jackson使用getter值。
• 默认情况下Jackson使用公共Getter可见性来访问该值。但是我们可以使用以下配置：

属性访问器.GETTER，可见性. PROTECTED_AND_PUBLIC）;
请参见示例：

public class ParentObj {
    private UUID id;    
    public ParentObj(UUID id) {
        this.id = id;       
    }   
    public UUID getUUID() {
        return id;
    }       
}
 Main.java
    ObjectMapper objectMapper = new ObjectMapper();
    System.out.println("ObjectMapper out > " + objectMapper.writeValueAsString(parentObj));

    Gson gson = new Gson();
    System.out.println("Gson out > " + gson.toJson(parentObj));

输出：

ObjectMapper out > {"uuid":"5d49e053-7954-4681-a3cc-1b5101d9712c"}
Gson out > {"id":"5d49e053-7954-4681-a3cc-1b5101d9712c"}