flagman — Sat, 12 Feb 2011 09:21:00 GMT

�?某网友讨论道: �?br>: RT�Q�反��的�Ҏ��发现很��用�?/em>

恰恰相反�Q�有些反��的�Ҏ��是�l�常会被使用到的�?/p>
反射��M��上分成两大特性，一是自省，二是发射�Q?/p>
自省的能力极为重要，而且几乎会天天用刎ͼ�很少见到�q�哪�?net应用中不使用attribute的，而attribute�Ҏ��就是metadata通过在自省能力支撑下实现的；当然自省不单单是attribute�Ҏ��的�q�用�Q�只要是在运行时动态检视程序自�w�的�Ҏ��都要由反射的自省能力来支持�Q�比如Visual Studio的IDE�Q�这个集成开发环境本�w�就�?net应用的好案例�Q�对�?net�l��g的自动探��功能；同时�Q�自省的能力也是��Z��虚拟机��^台的语言�Q�比如c#和java�Q�区别于传统语言比如c和c++的重要特性之一�Q�这提供了程序设计开发更��Z��利和安全的运行时环境�Q�相对而言�Q�在c++�Q�当然是native而不是managed�Q�的环境下，除了RTTI极�ؓ单薄的运行时自省�Q�也��是QT�q�个库通过meta-object system部分模拟了自省的�Ҏ��；

反射的另外一个重要特性就是发��，它让“�E�序可以写程�?#8221;了，��要的说就是在�q�行时动态生成MSIL�q�加载运行以及持久化动态生成的MSIL的能力；��p��个特性的支持�Q�让原先一些程序设计和开发领域相对困隑֒��J�琐的工作，比如元编�E�meta programming�Q�比如动态代理dynamic proxy�Q�比如AOP中的基础设施weaver的实玎ͼ�变得可能或相�Ҏ��于实玎ͼ�反射的特性，也是��Z��虚拟机��^台CLR的支持，以metadata为基��来实现的�Q�所以这也是虚拟机��^台语�a�的特有优势，而在传统语言�q�_��上，�q�是难以实现的；比如关于meta programming�Q�c++��是通过模板�Ҏ��实现的�~�译期meta programming�Q�这与虚拟机�q�_��上实现的�q�行时meta programming�q�是有比较大的差距（比如前者如何保证生成的代码的type-safe�Q�；

以上�q�两个特性，自省和发��，都有个共同点�Q�他们都是围�l�着metadata机制�Q��ƈ在虚拟机�q�_��q�行时环境CLR支持下实现的�Q�前者是�q�行时检视相关的metadata�Q�后者是�q�行时动态生成相关的metadata和MSIL�Q�从�q�点也就可以看出�Q�要��x��入理解这些特性，��需要研�I�metadata和MSIL的实玎ͼ�以及虚拟��行时环境的实玎ͼ�在java�q�_��上，��是bytecode和JVM�Q�；

所以，反射�Q�可能是虚拟机��^台所提供的相�Ҏ��为强�Ԍ��最为复杂，和��^台运行时本��n关系最密切�Q�也是区别于传统语言和运行时最鲜明的特性�?/p>

flagman 2011-02-12 17:21 发表评论

flagman — Mon, 13 Dec 2010 01:02:00 GMT

Fusion is one of the most importants features among ones in the runtime implementation of CLI.

In the fusion, or any other components or modules, how to retrieve the execution engine instance and how to generate such engine?

UtilExecutionEngine, implemented as COM object, support Queryinterface/AddRef/Release, and exposed via interface IExecutionEngine.

With SELF_NO_HOST defined,
BYTE g_ExecutionEngineInstance[sizeof(UtilExecutionEngine)];
g_ExecutionEngineInstance would be the singleton instance of current execution engine,

otherwise, without SELF_NO_HOST, the 'sscoree' dll would be loaded and try to get the exported function, which is named 'IEE' from such dll. Here, it is the well-known shim, in .net CLR, such module is named 'mscoree'. Further, if 'IEE' could not be found in such dll, system would try to locate another exported function, named 'LoadLibraryShim', and use such function to load the 'mscorwks' module, and try to locate the 'IEE' exportd functionin it.

It's very obvious that Rotor has implemented its own execution engine, but it also gives or make space for implementation of execution engine from 3rd party. Here, .net CLR is a good candidate definitely, Rotor might load the mscorwks.dll module for its usage.

PAL, PALAPI, for example, HeapAlloc, one famous WIN32 API, has been implemented as one PALAPI (defined in Heap.c), to make it possible that the CLI/Rotor be ported smoothly to other OS, such freebsd/mac os.

CRT routines are also reimplemented, such as memcpy, it has been implemented as GCSafeMemCpy

There're many macros in fuctions, such as SCAN_IGNORE_FAULT/STATIC_CONTRACT_NOTHROW/STATIC_CONTRACT_NOTRIGGER, they are for static analysis tool to scan, analyse and figour out the potential issues in code.

From view point of the execution model by CLI, the act of compiling (including JIT) high-level type descriptions would be separated from the act of turning these type descriptions into processor-specific code and memory structures.

And such executino model, in other word, the well-known 'managed execution', would defer the loading, verification and compilation of components until runtime really needs; At the same time, the type-loading is the key trigger that causes CLI's tool chain to be engaged at runtime. Deferred compilation(lead to JIT)/linking/loading would get better portability to different target platform and be ready for version change; The whole deferred process would driven by well-defined metadata and policy, and it would be very robust for building a virtual execution environment;

At the top of such CLI tool chain, fusion is reponsible for not only finding and binding related assemblies, which are via assembly reference defined in assembly, fusion also takes another important role, loader, and its part of functionality is implemented in PEAssembly, ClassLoader classes. For example, ClassLoader::LoadTypeHandleForTypeKey.

For types in virtual execution environment of CLI, rotor defines four kinds of elements for internal conducting,
ELEMENT_TYPE_CLASS for ordinary classes and generic instantiations(including value types);
ELEMENT_TYPE_ARRAY AND ELEMENT_TYPE_SZARRAY for array types
ELEMENT_TYPE_PRT and ELEMENT_TYPE_BYREF for pointer types
ELEMENT_TYPE_FNPTR for function pointer types

every type would be assigned unique ulong-typed token, and such token would be used to look up in m_TypeDefToMethodTableMap (Linear mapping from TypeDef token to MethodTable *)which is maintained by current module; If there it is, the pointer to method table of such type would be retrieved, or it would look up in the loader module, where the method table should exist in while it's JIT loaded, not launched from NGEN image;

And all the unresolved typed would be maintained in a hash table, PendingTypeLoadTable; Types and only those types that are needed, such as dependencies, including parent types, are loaded in runtime, such type is fully loaded and ready for further execution, and other unresolved types would be kept in the previous hash table.

flagman 2010-12-13 09:02 发表评论

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