/Prototipo/Servlet/lib/xstream-distribution-1.4.1-bin/xstream-1.4.1/docs/faq.html
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1<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 2<html xmlns="http://www.w3.org/1999/xhtml"> 3<!-- 4 Copyright (C) 2005, 2006 Joe Walnes. 5 Copyright (C) 2006, 2007, 2008 XStream committers. 6 All rights reserved. 7 8 The software in this package is published under the terms of the BSD 9 style license a copy of which has been included with this distribution in 10 the LICENSE.txt file. 11 12 Created on 29. January 2005 by Joe Walnes 13 --> 14 <head> 15 <title>XStream - Frequently Asked Questions</title> 16 <link rel="stylesheet" type="text/css" href="style.css"/> 17 18 19 20 21 <!-- Google analytics --> 22 <script src="http://www.google-analytics.com/urchin.js" type="text/javascript"> 23 </script> 24 <script type="text/javascript"> 25 _uacct = "UA-110973-2"; 26 urchinTracker(); 27 </script> 28 29 </head> 30 <body> 31 32 <div id="banner"> 33 <a href="index.html"><img id="logo" src="logo.gif" alt="XStream"/></a> 34 </div> 35 36 <div id="center" class="Content2Column"> <!-- Content3Column for index --> 37 <div id="content"> 38 <h1 class="FirstChild">Frequently Asked Questions</h1> 39 40 41 42 <ol> 43 <li><a href="#Compatibility">Compatibility</a></li> 44 <li><a href="#Serialization">Serialization</a></li> 45 <li><a href="#XML">XML specifics</a></li> 46 <li><a href="#JSON">JSON specifics</a></li> 47 <li><a href="#Other_Products">Comparison to other products</a></li> 48 <li><a href="#Scalability">Scalability</a></li> 49 <li><a href="#Uses">Uses of XStream</a></li> 50 </ol> 51 52 <!-- ****************************************************** --> 53 <h1 id="Compatibility">Compatibility</h1> 54 55 <!-- ...................................................... --> 56 <h2 id="Compatibility_JDK">Which JDK is required to use XStream?</h2> 57 <p>1.4 or later.</p> 58 59 <!-- ...................................................... --> 60 <h2 id="Compatibility_JVMs">Does XStream behave differently across different JVMs?</h2> 61 62 <p>XStream has two modes of operation: <b>Pure Java</b> and <b>Enhanced</b>. In pure Java mode, 63 XStream behaves in the same way across different JVMs, however its features are limited to what 64 reflection allows, meaning it cannot serialize certain classes or fields. In <b>enhanced</b> mode, 65 XStream does not have these limitations, however this mode of operation is not available to all JVMs.</p> 66 67 <!-- ...................................................... --> 68 <h2 id="Compatibility_enhanced_mode_jvm">Which JVMs allow XStream to operate in enhanced mode?</h2> 69 70 <p>Currently on the Sun, Apple, HP, IBM and Blackdown 1.4 JVMs and onwards. 71 Also for Hitachi, SAP and Diablo from 1.5 and onwards. 72 Support for BEA JRockit starting with R25.1.0. 73 OpenJDK 6 is also supported. 74 For all other JVMs, XStream should be used in pure Java mode.</p> 75 76 <!-- ...................................................... --> 77 <h2 id="Compatibility_enhanced_mode_advantage">What are the advantages of using enhanced mode over pure Java mode?</h2> 78 79 <table summary="Comparison of pure Java and enhanced mode"> 80 <tr><th>Feature</th><th>Pure Java</th><th>Enhanced Mode</th></tr> 81 <tr><td>Public classes</td><td>Yes</td><td>Yes</td></tr> 82 <tr><td>Non public classes</td><td>No</td><td>Yes</td></tr> 83 <tr><td>Static inner classes</td><td>Yes</td><td>Yes</td></tr> 84 <tr><td>Non-static inner classes</td><td>No</td><td>Yes</td></tr> 85 <tr><td>Anonymous inner classes</td><td>No</td><td>Yes</td></tr> 86 <tr><td>With default constructor</td><td>Yes</td><td>Yes</td></tr> 87 <tr><td>Without default constructor</td><td>No</td><td>Yes</td></tr> 88 <tr><td>Private fields</td><td>Yes</td><td>Yes</td></tr> 89 <tr><td>Final fields</td><td>Yes >= JDK 1.5</td><td>Yes</td></tr> 90 </table> 91 92 <!-- ...................................................... --> 93 <h2 id="Compatibility_XPP">Why is my application not able to create a XmlPullParser with the XppDriver since XStream 1.4?</h2> 94 95 <p>The <a href="http://www.xmlpull.org/">XML Pull Parser API</a> defines an own mechanism to load the factory for 96 the available XPP implementation. XStream's XppDriver never used this lookup mechanism automatically before version 97 1.4, now it will. Therefore you will have to add a <a href="download.html#optional-deps">dependency to xmlpull</a> 98 if the XPP implementation does not deliver the classes on its own. This dependency is necessary for Xpp3 in 99 contrast to kXML2 that contains the classes. Use the Xpp3Driver or the KXml2Driver if you want to select one of the 100 directly supported XPP implementation on your own without using the XPP factory. Note, that the minimal version of 101 kXML2 does not support the XPP factory, but can be used by the KXml2Driver.</p> 102 103 <!-- ...................................................... --> 104 <h2 id="Compatibility_Android">Can I use XStream in an Android application?</h2> 105 106 <p>XStream does work in Android 1.0, but is reported to have limited capabilities. Since XStream 1.4 Android is 107 treated at least as JD 5 platform, but it e.g. does not include the java.beans package. Therefore you cannot use 108 the JavaBeanConverter. Note, that Android provides an XML Pull Parser, therefore XStream can work without 109 additional dependencies.</p> 110 111 <!-- ...................................................... --> 112 <h2 id="Compatibility_Harmony">Why does XStream fail on Harmony?</h2> 113 114 <p>Since JDK 5 it is possible according the Java specification to write into final fields using reflection. This is not yet 115 supported by Harmony and therefore the PureJavaReflectionProvider fails. We have also already investigated into 116 enhanced mode in Harmony, but the Harmony JVM currently crashes running the unit tests. It is simply not yet ready.</p> 117 118 <!-- ...................................................... --> 119 <h2 id="Compatibility_unsupported_JVM">Are there plans to provide enhanced mode support to other JVMs?</h2> 120 <p>Yes. <a href="list-user.html">Let us know</a> which JVM you would like supported.</p> 121 122 <!-- ...................................................... --> 123 <h2 id="Compatibility_no_enhanced_mode">When should I use XStream not in enhanced mode?</h2> 124 125 <p>Running XStream in a secured environment can prevent XStream from running in enhanced mode. This is 126 especially true when running XStream in an applet. You may also try to use the JavaBeanConverter as alternative to 127 the ReflectionConverter running in enhanced or pure Java mode.</p> 128 129 <!-- ...................................................... --> 130 <h2 id="Compatibility_SecurityManager">Which permissions does XStream need when running with an active SecurityManager?</h2> 131 132 <p>This depends on the mode XStream is running in. Refer to the 133 <a href="http://svn.xstream.codehaus.org/browse/xstream/trunk/xstream/src/test/com/thoughtworks/acceptance/SecurityManagerTest.java">SecurityManagerTest</a> 134 for details.</p> 135 136 <!-- ...................................................... --> 137 <h2 id="Compatibility_XStream11">Why does XStream 1.2 no longer read XML generated with XStream 1.1.x?</h2> 138 139 <p>The architecture in XStream has slightly changed. Starting with XStream 1.2 the 140 <a href="javadoc/com/thoughtworks/xstream/io/HierarchicalStreamDriver.html">HierarchicalStreamDriver</a> 141 implementation is responsible to ensure that XML tags and attributes are valid names in XML, in XStream 1.1.x 142 this responsibility was part of the ClassMapper implementations. Under some rare circumstances this will result in 143 an unreadable XML due to the different processing order in the workflow of such problematic tag names.</p> 144 145 <p>You can run XStream in 1.1 compatibility mode though:</p> 146 147<div class="Source Java"><pre>XStream xstream = new XStream(new XppDriver(new XStream11XmlFriendlyReplacer())) { 148 protected boolean useXStream11XmlFriendlyMapper() { 149 return true; 150 } 151};</pre></div> 152 153 <!-- ...................................................... --> 154 <h2 id="Compatibility_ConverterAnnotations">XStream 1.3 ignores suddenly annotated converters (@XStreamConverter and @XStreamConverters)?</h2> 155 156 <p>XStream treats now all annotations the same and therefore it no longer auto-detects any annotation by 157 default. You can configure XStream to run in auto-detection mode, but be aware if the 158 <a href="annotations-tutorial.html#AutoDetect">implications</a>. As alternative you might register the 159 deprecated AnnotationReflectionConverter, that was used for XStream pre 1.3.x, but as drawback the functionality 160 to register a local converter with XStream.registerLocalConverter will no longer work.</p> 161 162 <!-- ...................................................... --> 163 <h2 id="Compatibility_element_sequence">XStream 1.3 suddenly has a different field order?</h2> 164 165 <p>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 166 <a href="javadoc/com/thoughtworks/xstream/io/HierarchicalStreamDriver.html">XStream12FieldKeySorter</a> that can be used to 167 <a href="#Serialization_sort_fields">sort the fields</a> according XStream 1.2.2.</p> 168 169 170 <!-- ****************************************************** --> 171 <h1 id="Serialization">Serialization</h1> 172 173 <!-- ...................................................... --> 174 <h2 id="Serialization_types">Which class types can be serialized by XStream?</h2> 175 176 <p>In contrast to the JDK XStream is not tied to a marker interface to serialize a class. XStream ships with some specialized <a href="converters.html">converters</a>, 177 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 178 the ones referred as POJO (Plain Old Java Object).</p> 179 180 <p>However, some types of classes exist with typical characteristics, that cannot be handled - at least not out of the box:</p> 181 182 <ol> 183 <li>Objects that are based on threads or thread local data: Thread, Timer, ThreadLocal and so on. These classes keep different data for different threads and there's 184 no possibility to recreate a thread in a generic way nor recreating thread specific data. There might be special use cases, but this will always involve a custom converter 185 where threads can be recreated in a specific way tied to that use case.</li> 186 <li>Class types that are based on generated classes. Such types have often names that are unique to the current process and will have no meaning 187 in a different process. A custom converter might help to write the appropriate data into the serializing stream to be able to recreate a equivalent class at deserialization 188 time.</li> 189 <li>Types that keep and use system resources like file handles, sockets, pipes and so on. ClassLoader, FileInputStream, FileOutputStream, Socket and so on. To 190 deserialize such a class the converter must be able to claim the appropriate resource from the system again. With the help of a custom converter this might be 191 possible, but with the reflection converter the deserialized class might refer a system resource that is no longer valid or belongs to somebody else. Behavior is 192 undefined then.</li> 193 <li>A very special case of such allocated system resources are those classes that keep handles to system memory directly, because they are partly implemented native. 194 It is known to be true for the Linux version of Sun's JDK that the BufferedImage references some system specific types of the JDK that themselves have member fields 195 with such memory handles. While it is possible at first sight to serialize and deserialize a BufferedImage, the reflection converter will also duplicate the memory handle. 196 As a result the JVM might crash easily because of freeing unallocated memory or freeing the same memory twice. It might be possible to create a custom converter, 197 but the data structure is really complex in this area and nobody has been investigating so far to such an extent. However, <strong>do not use the reflection converter 198 for these types! You have been warned!</strong></li> 199 <li>Inner class types of the JDK can often vary in implementation details between JDK versions and vendors and are therefore only compatible for the same JDK. This 200 includes collection types returned by the methods of the Collections class that wrap another one (like unmodifiableList) or the collections that are returned by the 201 different Map implementations for the keySet(), entrySet() and values() methods.</li> 202 </ol> 203 204 <!-- ...................................................... --> 205 <h2 id="Serialization_omit_field">How do I specify that a field should not be serialized?</h2> 206 <p>Make it <code>transient</code>, specify it with <code>XStream.omitField()</code> or 207 annotate it with @XStreamOmitField</p> 208 209 <!-- ...................................................... --> 210 <h2 id="Serialization_initialize_transient">How do I initialize a transient field at deserialization?</h2> 211 212 <p>XStream uses the same mechanism as the JDK serialization. Example:</p> 213 214<div class="Source Java"><pre>class ThreadAwareComponent { 215 private transient ThreadLocal component; 216 // ... 217 private Object readResolve() { 218 component = new ThreadLocal(); 219 return this; 220 } 221}</pre></div> 222 223 <!-- ...................................................... --> 224 <h2 id="Serialization_no_ctor_running">XStream is not calling the default constructor during deserialization.</h2> 225 226 <p>This is, in fact, the same case as above. XStream uses the same mechanism as the JDK serialization. When using 227 the enhanced mode with the optimized reflection API, it does not invoke the default constructor. The solution is to 228 implement the readResolve method as above:</p> 229 230<div class="Source Java"><pre>class MyExecutor { 231 private Object readResolve() { 232 // do what you need to do here 233 System.out.println("After instantiating MyExecutor"); 234 // at the end returns itself 235 return this; 236 } 237}</pre></div> 238 239 <!-- ...................................................... --> 240 <h2 id="Serialization_collections">What do serialized collections look like?</h2> 241 242 <p>See example for the <a href="converters.html#java.util">CollectionConverter</a>.</p> 243 <p>Note, that it is possible to configure XStream to omit the container element <i>toys</i> using implicit collections.</p> 244 245 <!-- ...................................................... --> 246 <h2 id="Serialization_Serializable">Do my classes have to implement Serializable if XStream is to serialize them?</h2> 247 <p>No.</p> 248 249 <!-- ...................................................... --> 250 <h2 id="Serialization_proxies">Can dynamic proxies be serialized?</h2> 251 <p>Yes.</p> 252 253 <!-- ...................................................... --> 254 <h2 id="Serialization_CGLIB">Can CGLIB proxies be serialized?</h2> 255 <p>Only limitedly. A proxy generated with the CGLIB Enhancer is supported, if the proxy uses either a factory or 256 only one callback. Then it is possible to recreate the proxy instance at unmarshalling time. Starting with XStream 1.3.1 257 CGLIB support is no longer automatically installed because of possible classloader problems and side-effects, 258 because of incompatible ASM versions. You can enable CGLIB support with:</p> 259<div class="Source Java"><pre>XStream xstream = new XStream() { 260 protected MapperWrapper wrapMapper(MapperWrapper next) { 261 return new CGLIBMapper(next); 262 } 263}; 264xstream.registerConverter(new CGLIBEnhancedConverter(xstream.getMapper(), xstream.getReflectionProvider())); 265</pre></div> 266 267 <!-- ...................................................... --> 268 <h2 id="Serialization_CGLIB_ExceptionInInitializerError">CGLIBEnhancedConverter fails at initialization with ExceptionInInitializerError</h2> 269 <p>This is not a problem of XStream. You have incompatible ASM versions in your classpath. CGLIB 2.1.x and below is based on 270 ASM 1.5.x which is incompatible to newer versions that are used by common packages like Hibernate, Groovy or Guice. Check 271 your dependencies and ensure that you are using either using cglib-nodep-2.x.jar instead of cglib-2.x.jar or update to 272 cglib-2.2.x that depends on ASM 3.1. However, the <em>nodep</em> version contains a copy of the ASM classes with private 273 packages and will therefore not raise class incompatibilities at all.</p> 274 275 <!-- ...................................................... --> 276 <h2 id="Serialization_CGLIB_2.0.1">Serialization fails with NoSuchMethodError: net.sf.cglib.proxy.Enhancer.isEnhanced(Ljava/lang/Class;)Z</h2> 277 <p>XStream uses this method to detect a CGLIB-enhanced proxy. Unfortunately the method is not available in the 278 cglib-2.0 version. Since this version is many years old and the method is available starting with cglib-2.0.1, please 279 consider an upgrade of the dependency, it works usually smoothly.</p> 280 281 <!-- ...................................................... --> 282 <h2 id="Serialization_Hibernate">How do I use XStream's Hibernate package to serialize my objects?</h2> 283 <p>Support of Hibernate enhanced collections and proxied types. To drop the internals of Hibernate when marshalling 284 such objects to XStream, all converters and the mapper has to be registered for the XStream instance:</p> 285<div class="Source Java"><pre>final XStream xstream = new XStream() { 286 protected MapperWrapper wrapMapper(final MapperWrapper next) { 287 return new HibernateMapper(next); 288 } 289}; 290xstream.registerConverter(new HibernateProxyConverter()); 291xstream.registerConverter(new HibernatePersistentCollectionConverter(xstream.getMapper())); 292xstream.registerConverter(new HibernatePersistentMapConverter(xstream.getMapper())); 293xstream.registerConverter(new HibernatePersistentSortedMapConverter(xstream.getMapper())); 294xstream.registerConverter(new HibernatePersistentSortedSetConverter(xstream.getMapper())); 295</pre></div> 296 297 <!-- ...................................................... --> 298 <h2 id="Serialization_system_attributes">My attributes are interpreted by XStream itself and cause unexpected behavior</h2> 299 <p>XStream's generic converters and the marshalling strategies use a number of attributes on their own. Especially the attributes named 300 <em>id</em>, <em>class</em> and <em>reference</em> are likely to cause such collisions. Main reason is XStream's history, because 301 originally user defined attributes were not supported and all attribute were system generated. Starting with XStream 1.3.1 you can redefine 302 those attributes to allow the names to be used for your own ones. The following snippet defines XStream to use different system attributes 303 for <em>id</em> and <em>class</em> while the field <em>id</em> of YourClass is written into the attribute <em>class</em>:</p> 304<div class="Source Java"><pre>XStream xstream = new XStream() { 305xstream.useAttributeFor(YourClass.class, "id"); 306xstream.aliasAttribute("class", "id"); 307xstream.aliasSystemAttribute("type", "class"); 308xstream.aliasSystemAttribute("refid", "id"); 309</pre></div> 310 311 <!-- ...................................................... --> 312 <h2 id="Serialization_sort_fields">Can I select the field order in which XStream serializes objects?</h2> 313 <p>Yes. XStream's ReflectionConverter uses the defined field order by default. You can override it by using an specific FieldKeySorter:</p> 314<div class="Source Java"><pre>SortableFieldKeySorter sorter = new SortableFieldKeySorter(); 315sorter.registerFieldOrder(MyType.class, new String[] { "firstToSerialize", "secondToSerialize", "thirdToSerialize" }); 316xstream = new XStream(new Sun14ReflectionProvider(new FieldDictionary(sorter))); 317</pre></div> 318 319 <!-- ...................................................... --> 320 <h2 id="Serialization_newer_class_versions">How does XStream deal with newer versions of classes?</h2> 321 322 <ul> 323 <li>If a new field is added to the class, deserializing an old version will leave the field uninitialized.</li> 324 <li>If a field is removed from the class, deserializing an old version that contains the field will cause an exception. 325 Leaving the field in place but declaring it as transient will avoid the exception, but XStream will not try to deserialize it.</li> 326 <li>If a class is renamed, aliases can be used to create an abstraction between the name used in XML and the real class name.</li> 327 <li>If a field is renamed, this should be treated as adding a field and removing a field.</li> 328 </ul> 329 330 <p>For more advanced class migrations, you may</p> 331 <ul> 332 <li>have to do custom pre-processing of the XML before sending it to XStream (for example, with XSLT or DOM manipulations)</li> 333 <li>declare new fields as transient</li> 334 <li>implement your own converter, that can handle the situation</li> 335 <li>add a readResolve() method to your class, that initializes the object accordingly</li> 336 <li>implement a custom mapper to ignore unknown fields automatically 337 (see acceptance test CustomMapperTest.testCanBeUsedToOmitUnexpectedElements())</li> 338 </ul> 339 340 <p>Future versions of XStream will include features to make these type of migrations easier.</p> 341 342 <!-- ...................................................... --> 343 <h2 id="Serialization_classloader">How does XStream cope with isolated class loaders?</h2> 344 345 <p>Serializing an object graph is never a problem, even if the classes of those objects have been loaded by 346 a different class loader. The situation changes completely at deserialization time. In this case you must set the 347 class loader to use with:</p> 348 349 <div class="Source Java"><pre>xstream.setClassLoader(yourClassLoader);</pre></div> 350 351 <p>Although XStream caches a lot of type related information to gain speed, it keeps those information in 352 tables with weak references that should be cleaned by the garbage collector when the class loader is freed.</p> 353 354 <p>Note, that this call should be made quite immediately after creating the XStream and before any other 355 configuration is done. Otherwise configuration based on special types might refer classes loaded with the wrong 356 classloader.</p> 357 358 <!-- ****************************************************** --> 359 <h1 id="XML">XML specifics</h1> 360 361 <!-- ...................................................... --> 362 <h2 id="XML_respect_encoding">Why does XStream not respect the encoding in the XML declaration?</h2> 363 364 <p>XStream architecture is based on IO Readers and Writers, while the XML declaration is the responsibility of XML 365 parsers. All <a href="javadoc/com/thoughtworks/xstream/io/HierarchicalStreamDriver.html">HierarchicalStreamDriver</a> 366 implementations respect the encoding since version 1.3, but only if you provide an 367 <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/io/InputStream.html">InputStream</a>. If XStream consumes a 368 <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/io/Reader.html">Reader</a> you have to initialize the reader with 369 the appropriate encoding yourself, since it is now the reader's task to perform the encoding and no XML parser can 370 change the encoding of a Reader and any encoding definition in the XML header will be ignored.</p> 371 372 <!-- ...................................................... --> 373 <h2 id="XML_write_XML_declaration">Why does XStream not write an XML declaration?</h2> 374 375 <p>XStream is designed to write XML snippets, so you can embed its output into an existing stream or string. 376 You can write the XML declaration yourself into the Writer before using it to call XStream.toXML(writer).</p> 377 378 <!-- ...................................................... --> 379 <h2 id="XML_write_UTF8">Why does XStream not write XML in UTF-8?</h2> 380 381 <p>XStream does no character encoding by itself, it relies on the configuration of the underlying XML writer. 382 By default it uses its own PrettyPrintWriter which writes into the default encoding of the current locale. To write 383 UTF-8 you have to provide a <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/io/Writer.html">Writer</a> 384 with the appropriate encoding yourself.</p> 385 386 <!-- ...................................................... --> 387 <h2 id="XML_double_underscores">Why do field names suddenly have double underscores in the generated XML?</h2> 388 389 <p>XStream maps Java class names and field names to XML tags or attributes. Unfortunately this mapping cannot 390 be 1:1, since some characters used for <a href="http://java.sun.com/docs/books/jls/third_edition/html/lexical.html#3.8"> 391 identifiers in Java</a> are invalid in <a href="http://www.w3.org/TR/REC-xml/#dt-name">XML names</a>. Therefore 392 XStream uses an <a href="javadoc/com/thoughtworks/xstream/io/xml/XmlFriendlyNameCoder.html">XmlFriendlyNameCoder</a> 393 to replace these characters with a replacement. By default this 394 <a href="javadoc/com/thoughtworks/xstream/io/naming/NameCoder.html">NameCoder</a> uses an underscore as escape 395 character and has therefore to escape the underscore itself also. You may provide a different configured instance 396 of the XmlFriendlyNameCoder or a complete different implementation like the 397 <a href="javadoc/com/thoughtworks/xstream/io/xml/XmlFriendlyNameCoder.html">NoNameCoder</a> to prevent name coding 398 at all. However it is your responsibility then to ensure, that the resulting names are valid for XML.</p> 399 400 <!-- ...................................................... --> 401 <h2 id="XML_unmarshalling_fails">XStream fails to unmarshal my given XML and I do not know why?</h2> 402 403 <p>By default XStream is written for persistence i.e. it will read the XML it can write. If you have to transform 404 a given XML into an object graph, you should go the other way round. Use XStream to transfer your objects into XML. 405 If the written XML matches your schema, XStream is also able to read it. This way is much easier, since you can 406 spot the differences in the XML much more easy than to interpret the exceptions XStream will throw if it cannot 407 match the XML into your objects.</p> 408 409 <!-- ...................................................... --> 410 <h2 id="XML_null_char">My parser claims the &#x0; character to be invalid, but it was written with XStream!</h2> 411 412 <p>Your parser is basically right! A character of value 0 is not valid as part of XML according the XML specification (see 413 version <a href="http://www.w3.org/TR/2006/REC-xml-20060816/#charsets">1.0</a> or 414 <a href="http://www.w3.org/TR/2006/REC-xml11-20060816/#charsets">1.1</a>), neither directly nor as character 415 entity nor within CDATA. But not every parser respects this part of the specification (e.g. Xpp3 will ignore it and read 416 character entities). If you expect such characters in your strings and you do not use the Xpp3 parser, you should consider 417 to use a converter that writes the string as byte array in Base64 code. As alternative you may force the 418 <a href="javadoc/com/thoughtworks/xstream/io/xml/PrettyPrintWriter.html">PrettyPrintWriter</a> or derived writers 419 to be XML 1.0 or 1.1. compliant, i.e. in this mode a StreamException is thrown.</p> 420 421 <!-- ...................................................... --> 422 <h2 id="XML_control_char">My parser claims a control character to be invalid, but it was written with XStream!</h2> 423 424 <p>Your parser is probably right! Control characters are only valid as part of XML 1.1. You should add an XML header 425 declaring this version or use a parser that does not care about this part of the specification (e.g. Xpp3 parser).</p> 426 427 <!-- ...................................................... --> 428 <h2 id="XML_attributes">Why is my element not written as XML attribute although I have configured it?</h2> 429 430 <p>You can only write types as attributes that are represented as a single String value and are handled therefore 431 by SingleValueConverter implementations. If your type is handled by a Converter implementation, the configuration 432 of XStream to write an attribute (using XStream.useAttributeFor() or @XStreamAsAttribute) is simply ignored.</p> 433 434 <!-- ...................................................... --> 435 <h2 id="XML_attribute_normalization">Why are whitespace characters missing in my attribute values after deserialization?</h2> 436 437 <p>This is part of the XML specification and a required functionality for any XML parser called 438 <a href="http://www.w3.org/TR/2006/REC-xml-20060816/#AVNormalize">attribute value normalization</a>. It cannot 439 be influenced by XStream. Do not use attributes if your values contain leading or trailing whitespaces, other whitespaces 440 than blanks, or sequences of whitespaces.</p> 441 442 <!-- ...................................................... --> 443 <h2 id="XML_namespaces">Why does XStream not have any namespace support?</h2> 444 445 <p>Not every XML parser supports namespaces and not every XML parser that supports namespaces can be configured 446 within XStream to use those. Basically namespaces must be supported individually for the different XML parsers and the 447 only support for namespaces that has currently been implemented in XStream is for the StAX paser. Therefore use and 448 configure the StaxDriver of XStream to use namespaces.</p> 449 450 <!-- ...................................................... --> 451 <h2 id="XML_xpath_limits">My XML contains XPath expressions in the references, but they seem not to work?</h2> 452 453 <p>XStream generates only XPath compliant expressions. These have a very limited syntax and they are the only ones 454 that can be interpreted at deserialization again, since XStream does not use an XPath interpreter. Therefore there 455 is no support for attribute selectors, qualified element access with axis names or functions. For real XPath 456 support you will have to implement your own MarshallingStrategy.</p> 457 458 <!-- ...................................................... --> 459 <h2 id="XML_xpath_nodelist">The XPath expressions in the references do select a list, but not a single node!</h2> 460 461 <p>Yes, this is right. However, the result type of an 462 <a href="http://java.sun.com/j2se/1.5.0/docs/api/javax/xml/xpath/XPathExpression.html">XPath expression</a> 463 evaluation can be defined. A node result from a node list is the lists first node, therefore the XPath of XStream 464 is compliant. Since XStream does not use a real XPath engine, you do not have to worry about memory consumption or 465 wasted evaluation time, XStream will always operate on a single node anyway. Since XStream 1.4 you can force 466 XStream to write XPath expressions that select explicit the single node by using the new modes 467 XStream.SINGLE_NODE_XPATH_ABSOLUTE_REFERENCES or SINGLE_NODE_XPATH_RELATIVE_REFERENCES. Instead of generating a 468 path like "/doc/list/elem/field" XStream will then generate "/doc[1]/list[1]/elem[1]/field[1]". The two notations 469 are transparent at deserialization time.</p> 470 471 472 <!-- ****************************************************** --> 473 <h1 id="JSON">JSON specifics</h1> 474 475 <h2 id="JSON_2_drivers">Why are there two JSON driver implementations?</h2> 476 477 <p>As always, first for historical reasons! Main difference is that the 478 <a href="javadoc/com/thoughtworks/xstream/io/json/JettisonMappedXmlDriver.html">JettisonMappedXmlDriver</a> is a 479 thin wrapper around <a href="http://jettison.codehaus.org">Jettison</a> in combination with the 480 <a href="javadoc/com/thoughtworks/xstream/io/xml/StaxDriver.html">StaxDriver</a>, while the 481 <a href="javadoc/com/thoughtworks/xstream/io/json/JsonHierarchicalStreamDriver.html">JsonHierarchicalStreamDriver</a> 482 uses an own more flexible implementation, but can only be used to generate JSON, deserialization is not implemented.</p> 483 484 <h2 id="JSON_Jettison_version">Which versions of Jettison are supported?</h2> 485 486 <p>Users of Java 5 or higher can use Jettison 1.2 or higher, users of Java 1.4.2 have to use Jettison 1.0.1. Java 487 1.3 and Jettison 1.1 is not supported.</p> 488 489 <h2 id="JSON_deserialize_top_level_array">Why is it not possible to deserialize a JSON string starting with an array?</h2> 490 491 <p>XStream's implementation to deserialize JSON is based on Jettison and StAX. Jettison implements a XMLStreamReader 492 of StaX and transforms the processed JSON virtually into XML first. However, if the JSON string starts with an array it is not 493 possible for Jettison to create a valid root element, since it has no name.</p> 494 495 <h2 id="JSON_unmarshalling_fails">XStream fails to unmarshal my JSON string and I do not know why?</h2> 496 497 <p>Deserialization of JSON is currently done by Jettison, that transforms the JSON string into a StAX stream. 498 XStream itself does nothing know about the JSON format here. If your JSON string reaches some kind of 499 complexity and you do not know how to design your Java objects and configure XStream to match those, 500 you should have a look at the intermediate XML that is processed by XStream in the end. This might help to 501 identify the problematic spots. Also consider then <a href="#XML_unmarshalling_fails">marshalling your Java 502 objects into XML first</a>. You can use following code to generate the XML:</p> 503 504<div class="Source Java"><pre>String json = "{\"string\": \"foo\"}"; 505HierarchicalStreamDriver driver = new JettisonMappedXmlDriver(); 506StringReader reader = new StringReader(json); 507HierarchicalStreamReader hsr = driver.createReader(reader); 508StringWriter writer = new StringWriter(); 509new HierarchicalStreamCopier().copy(hsr, new PrettyPrintWriter(writer)); 510writer.close(); 511System.out.println(writer.toString()); 512</pre></div> 513 514 <h2 id="JSON_limitations">What limitations has XStream's JSON support?</h2> 515 516 <p>JSON represents a very simple data model for easy data transfer. Especially it has no equivalent for XML 517 attributes. Those are written with a leading "@" character, but this is not always possible without 518 violating the syntax (e.g. for array types). Those may silently dropped (and makes it therefore difficult to 519 implement deserialization). References are another issue in the serialized object graph, since JSON has no 520 possibility to express such a construct. You should therefore always set the NO_REFERENCES mode of XStream. 521 Additionally you cannot use implicit collections, since the properties in a JSON object must have unique names.</p> 522 523 <h2 id="JSON_encoding">Why are there invalid characters in my JSON representation?</h2> 524 525 <p>The JSON spec requires any JSON string to be in UTF-8 encoding. However, XStream ensures this only if you 526 provide an InputStream or an OutputStream. If you provide a Reader or Writer you have to ensure this requirement 527 on your own.</p> 528 529 <h2 id="JSON_dashes">The generated JSON is invalid, it contains a dash in the label!</h2> 530 531 <p>Well, no, the JSON is valid! Please check yourself with the <a href="http://www.jslint.com/">JSON syntax checker</a>. 532 However, some JavaScript libraries silently assume that the JSON labels are valid JavaScript identifiers, because JavaScript 533 supports a convenient way to address an element, <strong>if</strong> the label is a valid JavaScript identifier:</p> 534 535<div class="Source JavaScript"><pre>var json = {"label": "foo", "label-with-dash": "bar"}; 536var fooVar = json.label; // works for labels that are JavaScript identifiers 537var barVar = json["label-with-dash"]; // using an array index works always 538</pre></div> 539 540 <p>As alternative you may wrap the JsonWriter and replace any dash with an underscore:</p> 541 542<div class="Source Java"><pre>HierarchicalStreamDriver driver = new JsonHierarchicalStreamDriver() { 543 public HierarchicalStreamWriter createWriter(Writer out) { 544 return new WriterWrapper(super.createWriter(out)) { 545 public void startNode(String name) { 546 startNode(name, null); 547 } 548 public void startNode(String name, Class clazz) { 549 wrapped.startNode(name.replace('-', '_'), clazz); 550 } 551 } 552 } 553}; 554XStream xstream = new XStream(driver); 555</pre></div> 556 557 <!-- ****************************************************** --> 558 <h1 id="Other_Products">Comparison to other products</h1> 559 560 <!-- ...................................................... --> 561 <h2 id="Other_Products_XMLBeanEncoder">How does XStream compare to java.beans.XMLEncoder?</h2> 562 563 <p>XStream is designed for serializing <i>objects</i> using internal fields, whereas 564 <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/beans/XMLEncoder.html">XMLEncoder</a> is designed for 565 serializing <i>JavaBeans</i> using public API methods (typically in the form 566 of <code>getXXX()</code>, <code>setXXX()</code>, <code>addXXX()</code> and <code>removeXXX()</code> methods.</p> 567 568 <h2 id="Other_Products_JAXB">How does XStream compare to JAXB (Java API for XML Binding)?</h2> 569 570 <p>JAXB is a Java binding tool. It generates Java code from a schema and you are able to transform from those classes into 571 XML matching the processed schema and back. Note, that you cannot use your own objects, you have to use what is 572 generated.</p> 573 574 <!-- 575 <h1>Comparison to other products</h1> 576 577 <h2>How does XStream compare to JAXB (Java API for XML Binding)?</h2> 578 <p>Todo...</p> 579 580 <h2>How does XStream compare to JSX?</h2> 581 <p>Todo...</p> 582 583 <h2>How does XStream compare to Betwixt?</h2> 584 <p>Todo...</p> 585 586 <h2>How does XStream compare to Castor?</h2> 587 <p>Todo...</p> 588 589 <h2>How does XStream compare to Electric XML?</h2> 590 <p>Todo...</p> 591 592 <h2>How does XStream compare to JOX?</h2> 593 <p>Todo...</p> 594 595 <h2>How does XStream compare to JIBX?</h2> 596 <p>Todo...</p> 597 598 --> 599 600 601 602 <!-- ****************************************************** --> 603 <h1 id="Scalability">Scalability</h1> 604 605 <!-- ...................................................... --> 606 <h2 id="Scalability_Thread_safety">Is XStream thread safe?</h2> 607 608 <p>Yes. Once the XStream instance has been created and configured, it may be shared across multiple threads 609 allowing objects to be serialized/deserialized concurrently (unless you enable the auto.detection and processing of 610 annotations). Actually the creation and initialization of XStream is quite expensive, therefore it is recommended to 611 keep the XStream instance itself.</p> 612 613 <!-- ...................................................... --> 614 <h2 id="Scalability_Memory">How much memory does XStream consume?</h2> 615 616 <p>This cannot be answered in general, but following topics have impact on the memory:</p> 617 618 <ol> 619 <li>XML parser technology in use: You should use a streaming parser like Xpp3 or StAX. DOM-based 620 parsers process the complete XML and create their document model in memory before the first converter of XStream 621 is called.</li> 622 <li>Your object model: Is it necessary to keep the complete object graph in memory at once. As alternative you might 623 use <a href="objectstream.html">object streams</a> or write custom converters that can load and save objects of your 624 object model on the fly without adding them to the object graph physically. As example see the implementation of the 625 <a href="javadoc/com/thoughtworks/xstream/persistence/XmlArrayList.html">XmlArrayList</a> in combination with the 626 <a href="javadoc/com/thoughtworks/xstream/persistence/FileStreamStrategy.html">FileStreamStrategy</a> to keep 627 parts of the object graph separate.</li> 628 <li>References: By default XStream supports references to the same object in an object graph. This implies that XStream 629 keeps track of all serialized and deserialized objects internally. These references are kept with WeakReferences, so that the 630 memory can be freed as soon as nobody references these objects anymore.</li> 631 <li>XML values: Any tag and attribute value that is converted into a Java String in the object graph will use the same String 632 instance.</li> 633 <li>XStream caches: To increase performance XStream caches quite a lot like classes, converters to use, aliasing, tag names. 634 All those caches make usage of WeakReferences or will exist only while marshalling one object graph resp. unmarshalling one 635 input stream.</li> 636 </ol> 637 638 <!-- ...................................................... --> 639 <h2 id="Scalability_Performance">Can the performance of XStream be increased?</h2> 640 641 <p>XStream is a generalizing library, it inspects and handles your types on the fly. Therefore it will normally be slower than 642 a piece of optimized Java code generated out of a schema. However, it is possible to increase the performance anyway:</p> 643 644 <ul> 645 <li>Write custom converters for those of your types that occur very often in your XML.</li> 646 <li>Keep a configured XStream instance for multiple usage. Creation and initialization is quite expensive compared to the 647 overhead of XStream when calling marshall or unmarshal.</li> 648 <li>Use Xpp3 or StAX parsers.</li> 649 </ul> 650 651 <p>Note, you should never try to optimize code for performance simply because you <strong>believe</strong> that you 652 have detected a bottle neck. Always use proper tools like a profiler to verify where your hotspots are and whether your 653 optimization was really successful or not.</p> 654 655 656 <!-- ****************************************************** --> 657 <h1 id="Uses">Uses of XStream</h1> 658 659 <!-- ...................................................... --> 660 <h2 id="Uses_Data_Binding">Is XStream a data binding tool?</h2> 661 662 <p>No. It is a serialization tool.</p> 663 664 <!-- ...................................................... --> 665 <h2 id="Uses_Generate_Code">Can XStream generate classes from XSD?</h2> 666 667 <p>No. For this kind of work a data binding tool such as <a href="http://xmlbeans.apache.org">XMLBeans</a> is appropriate.</p> 668 669 <!-- ...................................................... --> 670 <h2 id="Uses_No_SAX_Reader">Why is there no SaxReader?</h2> 671 672 <p>XStream works on a stream-based parser model, while SAX is event-based. The stream based model implies, that the 673 caller consumes the individual tokens from the XML parser on demand, while in an event-based model the parser 674 controls the application flow on its own and will use callbacks to support client processing. The different 675 architecture makes it therefore impossible for XStream to use an event-driven XML parser.</p> 676 677 678 679 <br/> 680 681 </div> 682 </div> 683 684 <div class="SidePanel" id="left"> 685 <div class="MenuGroup"> 686 <h1>Software</h1> 687 <ul> 688 <li><a href="index.html">About XStream</a></li> 689 <li><a href="news.html">News</a></li> 690 <li><a href="changes.html">Change History</a></li> 691 <li><a href="versioning.html">About Versioning</a></li> 692 </ul> 693 </div> 694 <div class="MenuGroup"> 695 <h1>Evaluating XStream</h1> 696 <ul> 697 <li><a href="tutorial.html">Two Minute Tutorial</a></li> 698 <li><a href="graphs.html">Object references</a></li> 699 <li><a href="manual-tweaking-output.html">Tweaking the Output</a></li> 700 <li><a href="license.html">License</a></li> 701 <li><a href="download.html">Download</a></li> 702 <li><a href="references.html">References</a></li> 703 <li><a href="parser-benchmarks.html">Parser Benchmarks</a></li> 704 <li><a href="http://www.ohloh.net/projects/3459">Code Statistics</a></li> 705 </ul> 706 </div> 707 <div class="MenuGroup"> 708 <h1>Using XStream</h1> 709 <ul> 710 <li><a href="architecture.html">Architecture Overview</a></li> 711 <li><a href="converters.html">Converters</a></li> 712 <li class="currentLink">Frequently Asked Questions</li> 713 <li><a href="list-user.html">Users' Mailing List</a></li> 714 <li><a href="issues.html">Reporting Issues</a></li> 715 </ul> 716 </div> 717 <div class="MenuGroup"> 718 <h1>Javadoc</h1> 719 <ul> 720 <li><a href="javadoc/index.html">XStream Core</a></li> 721 <li><a href="hibernate-javadoc/index.html">Hibernate Extensions</a></li> 722 <li><a href="benchmark-javadoc/index.html">Benchmark Module</a></li> 723 </ul> 724 </div> 725 <div class="MenuGroup"> 726 <h1>Tutorials</h1> 727 <ul> 728 <li><a href="tutorial.html">Two Minute Tutorial</a></li> 729 <li><a href="alias-tutorial.html">Alias Tutorial</a></li> 730 <li><a href="annotations-tutorial.html">Annotations Tutorial</a></li> 731 <li><a href="converter-tutorial.html">Converter Tutorial</a></li> 732 <li><a href="objectstream.html">Object Streams Tutorial</a></li> 733 <li><a href="persistence-tutorial.html">Persistence API Tutorial</a></li> 734 <li><a href="json-tutorial.html">JSON Tutorial</a></li> 735 </ul> 736 </div> 737 <div class="MenuGroup"> 738 <h1>Developing XStream</h1> 739 <ul> 740 <li><a href="how-to-contribute.html">How to Contribute</a></li> 741 <li><a href="list-dev.html">Developers' Mailing List</a></li> 742 <li><a href="team.html">Development Team</a></li> 743 <li><a href="repository.html">Source Repository</a></li> 744 <li><a href="http://bamboo.ci.codehaus.org/browse/XSTREAM">Continuous Integration</a></li> 745 </ul> 746 </div> 747 </div> 748 749 </body> 750</html>