1.3 or later.
XStream has two modes of operation: Pure Java and Enhanced. In pure Java mode, XStream behaves in the same way across different JVMs, however its features are limited to what reflection allows, meaning it cannot serialize certain classes or fields. In enhanced mode, XStream does not have these limitations, however this mode of operation is not available to all JVMs.
Currently on the Sun, Apple, HP, IBM and Blackdown 1.4 JVMs and onwards. Also for Hitachi, SAP and Diablo from 1.5 and onwards. Support for BEA JRockit starting with R25.1.0. For all other JVMs, XStream should be used in pure Java mode.
Feature | Pure Java | Enhanced Mode |
---|---|---|
Public classes | Yes | Yes |
Non public classes | No | Yes |
Static inner classes | Yes | Yes |
Non-static inner classes | No | Yes |
Anonymous inner classes | No | Yes |
With default constructor | Yes | Yes |
Without default constructor | No | Yes |
Private fields | Yes | Yes |
Final fields | Yes >= JDK 1.5 | Yes |
XStream does work in Android 1.0, but has limited capabilities. XStream does not support enhanced mode for Android. Additionally Android reports itself as implementation of JDK specification 0.9, so it is not possible to write into final fields. Android also does not include the java.beans package. Therefore you cannot use the JavaBeanConverter.
Since JDK 5 it is possible according the Java specification to write into final fields using reflection. This is not yet supported by Harmony and therefore the PureJavaReflectionProvider fails. We have also already investigated into enhanced mode in Harmony, but the Harmony JVM currently crashes running the unit tests. It is simply not yet ready.
Yes. Let us know which JVM you would like supported.
Running XStream in a secured environment can prevent XStream from running in enhanced mode. This is especially true when running XStream in an applet. You may also try to use the JavaBeanConverter as alternative to the ReflectionConverter running in enhanced or pure Java mode.
This depends on the mode XStream is running in. Refer to the SecurityManagerTest for details.
The architecture in XStream has slightly changed. Starting with XStream 1.2 the HierarchicalStreamDriver implementation is responsible to ensure that XML tags and attributes are valid names in XML, in XStream 1.1.x this responsibility was part of the ClassMapper implementations. Under some rare circumstances this will result in an unreadable XML due to the different processing order in the workflow of such problematic tag names.
You can run XStream in 1.1 compatibility mode though:
XStream xstream = new XStream(new XppDriver(new XStream11XmlFriendlyReplacer())) { protected boolean useXStream11XmlFriendlyMapper() { return true; } };
XStream treats now all annotations the same and therefore it no longer auto-detects any annotation by default. You can configure XStream to run in auto-detection mode, but be aware if the implications. As alternative you might register the deprecated AnnotationReflectionConverter, that was used for XStream pre 1.3.x, but as drawback the functionality to register a local converter with XStream.registerLocalConverter will no longer work.
Yes. This was announced with the last 1.2.x release and was done to support the type inheritance of XML schemas. However, XStream is delivered with the XStream12FieldKeySorter that can be used to sort the fields according XStream 1.2.2.
In contrast to the JDK XStream is not tied to a marker interface to serialize a class. XStream ships with some specialized converters, but will use reflection by default for "unknown" classes to examine, read and write the class' data. Therefore XStream can handle quite any class, especially the ones referred as POJO (Plain Old Java Object).
However, some types of classes exist with typical characteristics, that cannot be handled - at least not out of the box:
Make it transient
, specify it with XStream.omitField()
or
annotate it with @XStreamOmitField
XStream uses the same mechanism as the JDK serialization. Example:
class ThreadAwareComponent { private transient ThreadLocal component; // ... private Object readResolve() { component = new ThreadLocal(); return this; } }
This is, in fact, the same case as above. XStream uses the same mechanism as the JDK serialization. When using the enhanced mode with the optimized reflection API, it does not invoke the default constructor. The solution is to implement the readResolve method as above:
class MyExecutor { private Object readResolve() { // do what you need to do here System.out.println("After instantiating MyExecutor"); // at the end returns itself return this; } }
See example for the CollectionConverter.
Note, that it is possible to configure XStream to omit the container element toys using implicit collections.
No.
Yes.
Only limitedly. A proxy generated with the CGLIB Enhancer is supported, if the proxy uses either a factory or only one callback. Then it is possible to recreate the proxy instance at unmarshalling time. Starting with XStream 1.3.1 CGLIB support is no longer automatically installed because of possible classloader problems and side-effects, because of incompatible ASM versions. You can enable CGLIB support with:
XStream xstream = new XStream() { protected MapperWrapper wrapMapper(MapperWrapper next) { return new CGLIBMapper(next); } }; xstream.registerConverter(new CGLIBEnhancedConverter(xstream.getMapper(), xstream.getReflectionProvider()));
This is not a problem of XStream. You have incompatible ASM versions in your classpath. CGLIB 2.1.x and below is based on ASM 1.5.x which is incompatible to newer versions that are used by common packages like Hibernate, Groovy or Guice. Check your dependencies and ensure that you are using either using cglib-nodep-2.x.jar instead of cglib-2.x.jar or update to cglib-2.2.x that depends on ASM 3.1. However, the nodep version contains a copy of the ASM classes with private packages and will therefore not raise class incompatibilities at all.
XStream uses this method to detect a CGLIB-enhanced proxy. Unfortunately the method is not available in the cglib-2.0 version. Since this version is many years old and the method is available starting with cglib-2.0.1, please consider an upgrade of the dependency, it works usually smoothly.
XStream's generic converters and the marshalling strategies use a number of attributes on their own. Especially the attributes named id, class and reference are likely to cause such collisions. Main reason is XStream's history, because originally user defined attributes were not supported and all attribute were system generated. Starting with XStream 1.3.1 you can redefine those attributes to allow the names to be used for your own ones. The following snippet defines XStream to use different system attributes for id and class while the field id of YourClass is written into the attribute class:
XStream xstream = new XStream() { xstream.useAttributeFor(YourClass.class, "id"); xstream.aliasAttribute("class", "id"); xstream.aliasSystemAttribute("type", "class"); xstream.aliasSystemAttribute("refid", "id");
Yes. XStream's ReflectionConverter uses the defined field order by default. You can override it by using an specific FieldKeySorter:
SortableFieldKeySorter sorter = new SortableFieldKeySorter(); sorter.registerFieldOrder(MyType.class, new String[] { "firstToSerialize", "secondToSerialize", "thirdToSerialize" }); xstream = new XStream(new Sun14ReflectionProvider(new FieldDictionary(sorter)));
For more advanced class migrations, you may
Future versions of XStream will include features to make these type of migrations easier.
Serializing an object graph is never a problem, even if the classes of those objects have been loaded by a different class loader. The situation changes completely at deserialization time. In this case you must set the class loader to use with:
xstream.setClassLoader(yourClassLoader);
Although XStream caches a lot of type related information to gain speed, it keeps those information in tables with weak references that should be cleaned by the garbage collector when the class loader is freed.
Note, that this call should be made quite immediately after creating the XStream and before any other configuration is done. Otherwise configuration based on special types might refer classes loaded with the wrong classloader.
XStream architecture is based on IO Readers and Writers, while the XML declaration is the responsibility of XML parsers. XStream all HierarchicalStreamDriver implementation respect the encoding since version 1.3, but only if you provide an InputStream. If XStream consumes a Reader you have to initialize the reader with the appropriate encoding yourself, since it is now the reader's task to perform the encoding and no XML parser can change the encoding of a Reader and any encoding definition in the XML header will be ignored.
XStream is designed to write XML snippets, so you can embed its output into an existing stream or string. You can write the XML declaration yourself into the Writer before using it to call XStream.toXML(writer).
XStream does no character encoding by itself, it relies on the configuration of the underlying XML writer. By default it uses its own PrettyPrintWriter which writes into the default encoding of the current locale. To write UTF-8 you have to provide a Writer with the appropriate encoding yourself.
XStream maps Java class names and field names to XML tags or attributes. Unfortunately this mapping cannot be 1:1, since some identifiers of Java contain a dollar sign which is invalid in XML names. Therefore XStream uses an XmlFriendlyReplacer to replace this character with a replacement. By default this replacement uses an underscore and therefore the replacer must escape the underscore itself also. You may provide a different configured instance of the XmlFriendlyReplacer or a complete own implementation, but you must ensure, that the resulting names are valid for XML.
By default XStream is written for persistence i.e. it will read the XML it can write. If you have to transform a given XML into an object graph, you should go the other way round. Use XStream to transfer your objects into XML. If the written XML matches your schema, XStream is also able to read it. This way is much easier, since you can spot the differences in the XML much more easy than to interpret the exceptions XStream will throw if it cannot match the XML into your objects.
Your parser is basically right! A character of value 0 is not valid as part of XML according the XML specification (see version 1.0 or 1.1), neither directly nor as character entity nor within CDATA. But not every parser respects this part of the specification (e.g. Xpp3 will ignore it and read character entities). If you expect such characters in your strings and you do not use the Xpp3 parser, you should consider to use a converter that writes the string as byte array in Base64 code. As alternative you may force the PrettyPrintWriter or derived writers to be XML 1.0 or 1.1. compliant, i.e. in this mode a StreamException is thrown.
Your parser is probably right! Control characters are only valid as part of XML 1.1. You should add an XML header declaring this version or use a parser that does not care about this part of the specification (e.g. Xpp3 parser).
You can only write types as attributes that are represented as a single String value and are handled therefore by SingleValueConverter implementations. If your type is handled by a Converter implementation, the configuration of XStream to write an attribute (using XStream.useAttributeFor() or @XStreamAsAttribute) is simply ignored.
This is part of the XML specification and a required functionality for any XML parser called attribute value normalization. It cannot be influenced by XStream. Do not use attributes if your values contain leading or trailing whitespaces, other whitespaces than blanks, or sequences of whitespaces.
Not every XML parser supports namespaces and not every XML parser that supports namespaces can be configured within XStream to use those. Basically namespaces must be supported individually for the different XML parsers and the only support for namespaces that has currently been implemented in XStream is for the StAX paser. Therefore use and configure the StaxDriver of XStream to use namespaces.
As always, first for historical reasons! Main difference is that the JettisonMappedXmlDriver is a thin wrapper around Jettison in combination with the StaxDriver, while the JsonHierarchicalStreamDriver uses an own more flexible implementation, but can only be used to generate JSON, deserialization is not implemented.
XStream's implementation to deserialize JSON is based on Jettison and StAX. Jettison implements a XMLStreamReader of StaX and transforms the processed JSON virtually into XML first. However, if the JSON string starts with an array it is not possible for Jettison to create a valid root element, since it has no name.
Deserialization of JSON is currently done by Jettison, that transforms the JSON string into a StAX stream. XStream itself does nothing know about the JSON format here. If your JSON string reaches some kind of complexity and you do not know how to design your Java objects and configure XStream to match those, you should have a look at the intermediate XML that is processed by XStream in the end. This might help to identify the problematic spots. Also consider then marshalling your Java objects into XML first. You can use following code to generate the XML:
String json = "{\"string\": \"foo\"}"; HierarchicalStreamDriver driver = new JettisonMappedXmlDriver(); StringReader reader = new StringReader(json); HierarchicalStreamReader hsr = driver.createReader(reader); StringWriter writer = new StringWriter(); new HierarchicalStreamCopier().copy(hsr, new PrettyPrintWriter(writer)); writer.close(); System.out.println(writer.toString());
JSON represents a very simple data model for easy data transfer. Especially it has no equivalent for XML attributes. Those are written with a leading "@" character, but this is not always possible without violating the syntax (e.g. for array types). Those may silently dropped (and makes it therefore difficult to implement deserialization). References are another issue in the serialized object graph, since JSON has no possibility to express such a construct. You should therefore always set the NO_REFERENCES mode of XStream.
The JSON spec requires any JSON string to be in UTF-8 encoding. However, XStream ensures this only if you provide an InputStream or an OutputStream. If you provide a Reader or Writer you have to ensure this requirement on your own.
Well, no, the JSON is valid! Please check yourself with the JSON syntax checker. However, some JavaScript libraries silently assume that the JSON labels are valid JavaScript identifiers, because JavaScript supports a convenient way to address an element, if the label is a valid JavaScript identifier:
var json = {"label": "foo", "label-with-dash": "bar"}; var fooVar = json.label; // works for labels that are JavaScript identifiers var barVar = json["label-with-dash"]; // using an array index works always
As alternative you may wrap the JsonWriter and replace any dash with an underscore:
HierarchicalStreamDriver driver = new JsonHierarchicalStreamDriver() { public HierarchicalStreamWriter createWriter(Writer out) { return new WriterWrapper(super.createWriter(out)) { public void startNode(String name) { startNode(name, null); } public void startNode(String name, Class clazz) { wrapped.startNode(name.replace('-', '_'), clazz); } } } }; XStream xstream = new XStream(driver);
XStream is designed for serializing objects using internal fields, whereas
XMLEncoder is designed for
serializing JavaBeans using public API methods (typically in the form
of getXXX()
, setXXX()
, addXXX()
and removeXXX()
methods.
JAXB is a Java binding tool. It generates Java code from a schema and you are able to transform from those classes into XML matching the processed schema and back. Note, that you cannot use your own objects, you have to use what is generated.
Yes. Once the XStream instance has been created and configured, it may be shared across multiple threads allowing objects to be serialized/deserialized concurrently (unless you enable the auto.detection and processing of annotations). Actually the creation and initialization of XStream is quite expensive, therefore it is recommended to keep the XStream instance itself.
This cannot be answered in general, but following topics have impact on the memory:
XStream is a generalizing library, it inspects and handles your types on the fly. Therefore it will normally be slower than a piece of optimized Java code generated out of a schema. However, it is possible to increase the performance anyway:
Note, you should never try to optimize code for performance simply because you believe that you have detected a bottle neck. Always use proper tools like a profiler to verify where your hotspots are and whether your optimization was really successful or not.
No. It is a serialization tool.
No. For this kind of work a data binding tool such as XMLBeans is appropriate.
XStream works on a stream-based parser model, while SAX is event-based. The stream based model implies, that the caller consumes the individual tokens from the XML parser on demand, while in an event-based model the parser controls the application flow on its own and will use callbacks to support client processing. The different architecture makes it therefore impossible for XStream to use an event-driven XML parser.