How do multiple languages interact in one project?
Generally, any decently sized web project will use about five languages: HTML, CSS, Javascript, some kind of server-side “getting things done” language (ASP, JSP, CGI scripts with Perl, PHP, etc.), and some variant of SQL for database connectivity.
(This is, of course, hand-waving away the argument about whether or not HTML and CSS count as programming languages – I’m the “they are, but just not Turing-complete languages” camp, but that’s a whole other thread.)
Some examples of how all those work together:
If you’re going the best-practices route, the structure of a web page is in HTML, and the instructions for how to display it are in CSS – which could be in the same file, but don’t have to be. The CSS contains a bunch of classes, which the HTML refers to, and it’s up to the browser to figure out how to click them together.
Taking all that a step further, any javascript scripts on that page can alter any of the HTML/CSS that is present (change contents of HTML entities, swap out one CSS class for another, change the behavior of the CSS, and so on.) It does this via something called the Document Object Model, which is essentially a language and platform-independent API to manipulate HTML pages in an object-like manner (at which point I’ll back away slowly and just provide a link to the relevant wiki article.)
But then, where does all the HTML / CSS / Javascript come from? That’s what the server-side language does. In the simplest form, the serer-side language is a program that returns a giant string holding an HTML page as its output. This, obviously, can get much more complex: HTML forms and query string parameters can be used as input for our server side program, and then you have the whole AJAX thing where the javascript gets to send data directly to the server language as well. You can also get fancy where the server language can customize the HTML, CSS, and Javascript that gets spit out – essentially, you have a program in one language writing a program in another language.
The Server-side language to SQL connection works much the same. There are a lot of ways to make it both more complex and safer, but the simplest way is for your server language to dynamically build a string with a SQL command in it, hand that to the database via some kind of connector, and get back a result set. (This is a case where you really do have a function that boils down to someValue = database.executeThisSQLCommand( SQLString ). )
So to wrap this up, different languages in this case either communicate by actually writing programs in each other, or by handing data around in very simple easy to parse formats that everybody can understand. (Strings, mainly.)
Having multiple languages in one project is actually quite common, however the principles behind are not always simple.
In the simple case, different languages are compiled to the same code. For example, C and C++ code typically is compiled into machine assembler or C# and VB.Net is compiled into IL (the language understood by the .NET runtime).
It gets more difficult if the languages/compilers use a differnt type system. There can be many different ways, basic data types such as integer, float and doubles are represented internally, and there is even more ways to represent strings. When passing types around between the different languages it must be sure that both sides interpret the type the same or - if not - the types are correctly mapped. This sort of type mapping is also known as marshalling.
Classic examples of interoperability between different program languages are (mostly from the Windows world):
- The various languages available for the .NET platfrom. This includes C#, VB.Net, J#, IronRuby, F#, XSLT and many other less popular languages.
- Native COM components written in C++ or VB can be used with a huge variety of languages: VBScript, VB, all .NET languages, Java
- Win32 api functions can be called from .NET or VB
- IPC (inter process communication)
- Corba, probably the most comprehensive (and most complex) approach
- Web services and other service-oriented architectures, probably the most modern approach
Multiple languages in use is called "interoperability" or "interop" for short.
Your example is wrong. Java can call C functions.
The language provides a mechanism for interoperability.
In the case of .NET, languages are compiled into IL as part of the CLI. Thus any .NET language can interop (call methods defined by) modules defined in any other .NET language.
As an example:
I can define a method in C#
static void Hello(){ Console.WriteLine("Hello World");}
And I can call it from Python (IronPython)
Hello()
And get the expected output.
Generally speaking, some languages interop better than others, especially if the language authors specifically made interop a feature of the language.
Multiple languages can interact with:
- Piped input/output (ANY language can do this because input and output must by necessity be implemented in every non-toy language)
- Having code in one language compile to a native library while the other supports calling native code.
- Communicating over a loopback network connection. You can run into difficulties with firewall interference this way.
- Databases. These can be thought of as a "universal" data storage format, and hence can be accessed by most languages with database extensions. This generally requires one program to finish operation before the next program can access the database. In addition, all 'communications' are generally written to disk.
- If the languages involved run on the same runtime (i.e. .NET, JVM), then you generally can pass object data from one language directly to the other with little impedence.
In almost every case, you have to convert any communication to a common format before it can be exchanged (the exception is languages on the same runtime). This is why multiple languages are rarely used in one project.