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| |
| <h1>Axis2 RPC Support</h1> |
| <p>This document describes Axis2's Remote Procedure Call support in |
| a set of easy to understand implementation steps.</p> |
| <div class="section"><h2>Introduction<a name="Introduction"></a></h2> |
| <p>Axis2 Remote Procedure Call (RPC) support may seem somewhat |
| tricky and confusing at first glance. However, Axis2 RPC strategy |
| is based on a set of well defined rules. This document aims to |
| drill down to the details of the strategy and resolve most of the |
| unknown bits and pieces. Note that Axis2 currently does not support |
| the rpc/encoded style. But it fully supports the rpc/literal |
| style.</p> |
| <p>We will discuss the Axis2 RPC strategy in the following |
| steps</p> |
| <div class="section"><h2>Step 1 - Converting RPC Style WSDL's into Doc/Lit Style |
| WSDL<a name="Step_1_-_Converting_RPC_Style_WSDLs_into_DocLit_StyleWSDL"></a></h2> |
| <p>This is probably the most confusing part of the RPC strategy. |
| Since the Axis2 code generator is based on pure doc/lit style, the |
| first step of the code generation process is to generate a wrapper |
| schema. This wrapper generation can be easily explained by using an |
| example.</p> |
| <p>Take the following piece of WSDL</p> |
| <div><pre> |
| ..... |
| <message name="requestMessage"> |
| <part name="part1" type="xs:string"/> |
| <part name="part2" type="xs:int"/> |
| </message> |
| <message name="responseMessage"> |
| <part name="part1" type="xs:string"/> |
| </message> |
| <portType name="echoPortType"> |
| <operation name="echo"> |
| <input message="y:requestMessage"/> |
| <output message="y:responseMessage"/> |
| </operation> |
| </portType> |
| <binding name="echoBinding" type="y:echoPortType"> |
| <soap:binding style="rpc" transport="http://schemas.xmlsoap.org/soap/http"/> |
| <operation name="echo"> |
| <soap:operation soapAction="echo"/> |
| <input> |
| <soap:body use="literal"/> |
| </input> |
| <output> |
| <soap:body use="literal"/> |
| </output> |
| </operation> |
| </binding> |
| ..... |
| </pre></div> |
| <p>The binding says rpc/lit is required and in this case the |
| message parts will need wrapping in the following order:</p> |
| <ol style="list-style-type: decimal"> |
| <li>The first element needs to have the operation name as the local |
| name and the operation namespace. (This happens to be the namespace |
| of the porttype - in this case the targetnamespace of the |
| WSDL.)</li> |
| <li>The children of this element are non namespace qualified |
| elements with the part names as local names (referred to as |
| <b>part element</b>)</li> |
| <li>In case the part refers to a standard type like the example |
| WSDL, the content of the part element would be of that type. If the |
| part refers to a complex type defined in the schema, the content of |
| the part element becomes of that type. Having an element reference |
| in the part when the binding is rpc is invalid.</li> |
| </ol> |
| <p>For example, the input wire message for the echo operation |
| mentioned in the above WSDL fragment would look like this:</p> |
| <div><pre> |
| <op:<b>echo</b> xmlns:op="porttype namespace"> |
| <<b>part1</b>>Hello World</part1> |
| <<b>part2</b>>123</part2> |
| </op:echo> |
| </pre></div> |
| <p>Note that the element name is in bold. The first one is the |
| operation name, the second and third are part names. It can be seen |
| that it is possible to generate a schema representing this |
| structure, and then treat the whole service as a pure doc/lit |
| service. In this case, the following piece of schema is generated |
| to make the rpc to doc conversion. Note that in this case the wire |
| message stays unchanged. It is only a different WSDL authoring |
| style</p> |
| <div><pre> |
| <xs:element name="echo"> |
| <xs:complexType> |
| <xs:sequence> |
| <xs:element name="part1" type="xs:string" /> |
| <xs:element name="part2" type="xs:int" /> |
| </xs:sequence> |
| </xs:complexType> |
| </xs:element> |
| </pre></div> |
| <p>What the Axis2 code generator does is exactly this. By looking |
| at the binding style, it generates a wrapper schema in places |
| required before handing over the Axis* hierarchy to the code |
| generator engine. In every case (even when the schema needs to be |
| unwrapped) this wrapping part will take place!</p> |
| <div class="section"><h2>Step 2 - Unwrapping the Schema<a name="Step_2_-_Unwrapping_the_Schema"></a></h2> |
| <p>If the schema needs to be unwrapped, it brings up a few issues. |
| This is mainly because the only thing that the emitters rely on |
| when generating code is a mapping table.</p> |
| <ol style="list-style-type: decimal"> |
| <li>When the schema is unwrapped, where will the unwrapping |
| information remain? |
| <p>There has to be a store to keep the information seperated. The |
| Axis * hierarchy can be used for this. It has nicely separated |
| information holders and a parameter store that can hold an |
| information bean.</p> |
| </li> |
| <li>How do we maintain uniqueness among message part names? |
| <p>Part names are only unique across a message and not globally. |
| However, due to the fact that we have a global mapping table, we |
| need a way to differentiate between parts of different messages. |
| The technique used here is to generate a QName that has the |
| operation name as a namespace and a suffix (like _input) appended |
| to the local name.</p> |
| </li> |
| </ol> |
| <p>Given these solutions, the first step in unwrapping is to walk |
| the schema and figure out the unwrappable items. The key player of |
| the unwrapping process is the unwrapping extension. It walks a |
| given schema and figure out the unwrappable parts if there are |
| any.</p> |
| <p>The current unwrapper looks for the following patterns and fails |
| if it is not found!</p> |
| <div><pre> |
| < element > |
| < complexType > |
| < sequence > |
| < element /> |
| < /sequence > |
| < /complexType > |
| < /element > |
| |
| </pre></div> |
| <p>Once this pattern is detected, the unwrapper details will be |
| added to the relevant AxisMessage component.</p> |
| <div class="section"><h2>Step 3 - Populate Type Information<a name="Step_3_-_Populate_Type_Information"></a></h2> |
| <p>The next step is to populate the Type information for the parts. |
| This has to be explicitly done by the data binding extensions, and |
| currently the ADB and XMLbeans extensions populate the relevant |
| AxisMessage by looking up their generated type systems. This type |
| information goes into the AxisMessage inside a |
| MessagePartInformationHolder instance.</p> |
| <p>The following code fragment from the ADB extension shows how the |
| AxisMessages get populated with the relevant type information. The |
| code is almost the same for the XMLBeans extension. Note the items |
| in bold.</p> |
| <div><pre> |
| if (message.getParameter(Constants.UNWRAPPED_KEY) != null) { |
| XmlSchemaType schemaType = message.getSchemaElement().getSchemaType(); |
| if (schemaType instanceof XmlSchemaComplexType) { |
| XmlSchemaComplexType cmplxType = (XmlSchemaComplexType) schemaType; |
| XmlSchemaParticle particle = cmplxType.getParticle(); |
| if (particle instanceof XmlSchemaSequence) { |
| XmlSchemaObjectCollection items = |
| ((XmlSchemaSequence) particle).getItems(); |
| for (Iterator i = items.getIterator(); i.hasNext();) { |
| Object item = i.next(); |
| if (item instanceof XmlSchemaElement) { |
| XmlSchemaElement xmlSchemaElement = (XmlSchemaElement) item; |
| XmlSchemaType eltSchemaType = xmlSchemaElement.getSchemaType(); |
| if (eltSchemaType != null) { |
| <b>populateClassName(eltSchemaType,mapper,opName,xmlSchemaElement.getName());</b> |
| } else if (xmlSchemaElement.getSchemaTypeName() != null) { |
| eltSchemaType = findSchemaType(schemaMap, |
| xmlSchemaElement.getSchemaTypeName()); |
| if (eltSchemaType!=null){ |
| populateClassName(eltSchemaType,mapper,opName,xmlSchemaElement.getName()); |
| } |
| } |
| } |
| } |
| } |
| } |
| } |
| </pre></div> |
| <p>The populateClassName looks like this</p> |
| <div><pre> |
| private static void populateClassName(XmlSchemaType eltSchemaType, |
| TypeMapper typeMap, |
| String opName, |
| String partName) { |
| Map metaInfoMap = eltSchemaType.getMetaInfoMap(); |
| if (metaInfoMap != null) { |
| <b>String className = (String) metaInfoMap. |
| get(SchemaConstants.SchemaCompilerInfoHolder.CLASSNAME_KEY); |
| QName partQName = WSDLUtil.getPartQName(opName, |
| WSDLConstants.INPUT_PART_QNAME_SUFFIX, |
| partName); |
| typeMap.addTypeMappingName(partQName,className);</b> |
| if (Boolean.TRUE.equals( |
| metaInfoMap.get(SchemaConstants. |
| SchemaCompilerInfoHolder.CLASSNAME_PRIMITVE_KEY))){ |
| //this type is primitive - add that to the type mapper status |
| //for now lets add a boolean |
| typeMap.addTypeMappingStatus(partQName,Boolean.TRUE); |
| } |
| |
| } |
| } |
| </pre></div> |
| <div class="section"><h2>Step 4 - Generate Code with Unwrapped Parameters<a name="Step_4_-_Generate_Code_with_Unwrapped_Parameters"></a></h2> |
| <p>The next step is generating the actual code. The |
| AxisServiceBasedMultiLanguageEmitter has a method that generates |
| the XML model for the input parameters, and that method includes |
| the relevant part parameters inside the relavant top level input |
| parameter element.</p> |
| <p>The relevant part of the XML model looks like this. Note that |
| this intermediate XML model is the one that is parsed against the |
| Stylesheets to generate the code.</p> |
| <div><pre> |
| <input> |
| <param name="param4" type="com.example.www.ServiceNameStub.Echo" shorttype="Echo" value="null" location="body" opname="echo"> |
| <param name="param5" type="java.lang.String" shorttype="String" value="null" location="body" opname="echo" partname="Part1" |
| primitive="yes"/> |
| <param name="param6" type="int" shorttype="int" value="0" location="body" opname="echo" partname="Part2" primitive="yes"/> |
| </param> |
| </input> |
| </pre></div> |
| <p>The next part is handled by the template. Basically, the |
| template looks after the generation of multiple parameters into the |
| method signatures, and then the generating of the relevant |
| serialization and deserialization code for the parameters.</p> |
| <div class="section"><h2>Bringing the Parameters Together and Exploding Them<a name="Bringing_the_Parameters_Together_and_Exploding_Them"></a></h2> |
| <p>This is a somewhat controversial area. The current Axis2 code |
| generator does the wrapping and unwrapping at the object level and |
| not the XML level. In short, the exploded parameters are only a |
| convenience and the explosion does not run down to the XML level. |
| The following example of generated source code makes this |
| clear:</p> |
| <div><pre> |
| private org.apache.axiom.soap.SOAPEnvelope toEnvelope( |
| org.apache.axiom.soap.SOAPFactory factory, java.lang.String param1, |
| int param2, boolean optimizeContent) { |
| <b>com.example.www.ServiceNameStub.Echo wrappedType = new com.example.www.ServiceNameStub.Echo(); |
| wrappedType.setPart1(param1); |
| wrappedType.setPart2(param2);</b> |
| rg.apache.axiom.soap.SOAPEnvelope emptyEnvelope = factory.getDefaultEnvelope(); |
| emptyEnvelope.getBody().addChild(wrappedType.getOMElement( |
| com.example.www.ServiceNameStub.Echo.MY_QNAME, factory)); |
| |
| return emptyEnvelope; |
| } |
| </pre></div> |
| <p>Note the lines in bold. The wrapped class will anyway be |
| instantiated and used at the end, but what the user sees is |
| different. Exploding the parameters happens in a similar way!</p> |
| <div class="section"><h2>Conclusion<a name="Conclusion"></a></h2> |
| <p>Axis2 RPC support is sort of a misty area, but it is based on a |
| well defined set of rules which makes it not <i>that</i> misty |
| after all!</p> |
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