Marshalling (computer science)
In computer science, marshalling or marshaling is the process of transforming the memory representation of an object to a data format suitable for storage or transmission, and it is typically used when data must be moved between different parts of a computer program or from one program to another. Marshalling is similar to serialization and is used to communicate to remote objects with an object, in this case a serialized object. It simplifies complex communication, using composite objects in order to communicate instead of primitives. The inverse of marshalling is called unmarshalling.
Usage
Marshalling is used within implementations of different remote procedure call mechanisms, where it is necessary to transport data between processes and/or between threads. In Microsoft's Component Object Model, interface pointers must be marshalled when crossing COM apartment boundaries. In the.NET Framework, the conversion between an unmanaged type and a CLR type, as in the P/Invoke process, is also an example of an action that requires marshalling to take place.Additionally, marshalling is used extensively within scripts and applications that use the XPCOM technologies provided within the Mozilla application framework. The Mozilla Firefox browser is a popular application built with this framework, that additionally allows scripting languages to use XPCOM through XPConnect.
Example
In the Microsoft Windows family of operating systems the entire set of device drivers for Direct3D are kernel-mode drivers. The user-mode portion of the API is handled by the DirectX runtime provided by Microsoft.This is an issue because calling kernel-mode operations from user-mode requires performing a system call, and this inevitably forces the CPU to switch to "kernel mode". This is a slow operation, taking on the order of microseconds to complete. During this time, the CPU is unable to perform any operations. As such, minimizing the number of times this switching operation must be performed would optimize performance to a substantive degree.
Linux OpenGL drivers are split in two: a kernel-driver and a user-space driver. The user-space driver does all the translation of OpenGL commands into machine code to be submitted to the GPU. To reduce the number of system calls, the user-space driver implements marshalling. If the GPU's command buffer is full of rendering data, the API could simply store the requested rendering call in a temporary buffer and, when the command buffer is close to being empty, it can perform a switch to kernel-mode and add a number of stored commands all at once.
Comparison with serialization
To "serialize" an object means to convert its state into a byte stream in such a way that the byte stream can be converted back into a copy of the object.The term "marshal" is used for a specific type of "serialization" in the Python standard library - storing internal python objects:
In the Java-related RFC 2713, marshalling is used when serialising objects for remote invocation:
In Microsoft.NET, marshalling is also used to refer to serialization when using remote calls:
Unmarshalling
In computer science, unmarshalling or demarshaling refers to the process of transforming a representation of an object that was used for storage or transmission to a representation of the object that is executable. A serialized object which was used for communication can not be processed by a computer program. An unmarshalling interface takes the serialized object and transforms it into an executable form. Unmarshalling is the reverse process of marshalling.Usage
Usually XML objects are used when data needs to be transferred between processes, threads or systems, because this results in shorter message wire format and efficient data transfers. Once the data is transferred back to a program or an application, it needs to be converted back to an executable object for usage. Hence, unmarshalling is generally used in the receiver end of the implementations of Remote Method Invocation and Remote Procedure Call mechanisms to unmarshal transmitted objects in an executable form.JAXB
or Java Architecture for XML Binding is the most common framework used by developers to marshal and unmarshal Java objects. JAXB provides for the interconversion between fundamental data types supported by Java and standard XML schema data types.XmlSerializer
XmlSerializer is the framework used by C# developers to marshal and unmarshal C# objects. One of the advantages of C# over Java is that C# natively supports marshalling due to the inclusion of XmlSerializer class. Java, on the other hand requires a non-native glue code in the form of JAXB to support marshalling.XML and Executable Representation
An example of unmarshalling is the conversion of an XML representation of an object to the default representation of the object in any programming language. Consider the following class.public class Student
- XML representation of Student object:
- Executable representation of Student object:
//Code Snippet: 2
Student s1 = new Student;
s1.setID;
s1.setName;
Student s2 = new Student;
s2.setID;
s2.setName;
Unmarshaller in JAXB
The process of unmarshalling the XML data into an executable Java object is taken care of by the in-built Unmarshaller class. It also validates the XML data as it gets unmarshalled. The unmarshal methods defined in the Unmarshaller class are overloaded for the different types of XML inputs. Some of the important implementations of unmarshal methods:- Unmarshalling from an XML File:
JAXBContext jcon = JAXBContext.newInstance;
Unmarshaller umar = jcon.createUnmarshaller;
Object obj = umar.unmarshal;
- Unmarshalling from an XML file in InputStream:
InputStream istr = new FileInputStream;
JAXBContext jcon = JAXBContext.newInstance;
Unmarshaller umar = jcon.createUnmarshaller;
Object obj = umar.unmarshal;
- Unmarshalling from an XML file in a URL:
JAXBContext jcon = JAXBContext.newInstance;
Unmarshaller umar = jcon.createUnmarshaller;
URL url = new URL;
Object obj = umar.unmarshal;
Unmarshalling XML Data
Unmarshal methods can deserialize an entire XML document or a small part of it. When the XML root element is globally declared, these methods utilize the JAXBContext's mapping of XML root elements to JAXB mapped classes to initiate the unmarshalling. If the mappings are not sufficient and the root elements are declared locally, the unmarshal methods use declaredType methods for the unmarshalling process. These two approaches can be understood below.Unmarshal a global XML root element
The unmarshal method uses JAXBContext to unmarshal the XML data, when the root element is globally declared. The JAXBContext object always maintains a mapping of the globally declared XML element and its name to a JAXB mapped class. If the XML element name or its@xsi:type attribute matches the JAXB mapped class, the unmarshal method transforms the XML data using the appropriate JAXB mapped class. However, if the XML element name has no match, the unmarshal process will abort and throw an UnmarshalException. This can be avoided by using the unmarshal by declaredType methods.Unmarshal a local XML root element
When the root element is not declared globally, the application assists the unmarshaller by application-provided mapping using declaredType parameters. By an order of precedence, even if the root name has a mapping to an appropriate JAXB class, the declaredType overrides the mapping. However, if the@xsi:type attribute of the XML data has a mapping to an appropriate JAXB class, then this takes precedence over declaredType parameter. The unmarshal methods by declaredType parameters always return a JAXBElement instance. The properties of this JAXBElement instance are set as follows:| JAXBElement Property | Value |
| name | xml element name |
| value | instanceof declaredType |
| declaredType | unmarshal method declaredType parameter |
| scope | null |
Comparison with deserialization
An object that is serialized is in the form of a byte stream and it can eventually be converted back to a copy of the original object. Deserialization is the process of converting the byte stream data back to its original object type.An object that is marshalled, however, records the state of the original object and it contains the codebase. Hence, in order to convert the object state and codebase, unmarshalling must be done. The unmarshaller interface automatically converts the marshalled data containing codebase into an executable Java object in JAXB. Any object that can be deserialized can be unmarshalled. However, the converse need not be true.