{"id":16,"date":"1999-06-06T04:08:00","date_gmt":"1999-06-06T04:08:00","guid":{"rendered":""},"modified":"2020-08-01T20:55:02","modified_gmt":"2020-08-01T20:55:02","slug":"growing-frameworks-plugins","status":"publish","type":"post","link":"https:\/\/billwake.com\/growing-frameworks-plugins\/","title":{"rendered":"Growing Frameworks: Plugins"},"content":{"rendered":"
What<\/div>\n
 <\/div>\n
Why<\/div>\n
               <\/span> vs external program<\/div>\n
               <\/span> vs source code<\/div>\n
 <\/div>\n
Mechanics of plugin client<\/div>\n
               <\/span> Code to interface<\/div>\n
               <\/span> Move class files to plugin directory<\/div>\n
               <\/span> Watch it work<\/div>\n
 <\/div>\n
Mechanics of server<\/div>\n
               <\/span> Define interface<\/div>\n
               <\/span> Define directory<\/div>\n
               <\/span> Load classes<\/div>\n
               <\/span> Instantiate & call<\/div>\n
 <\/div>\n
Plugins in Java<\/div>\n
 <\/div>\n
Consequences<\/div>\n
               <\/span> Security<\/div>\n
               <\/span> Security Manager<\/div>\n
               <\/span> CPU<\/div>\n
 <\/div>\n
Applications in the Site Manager<\/div>\n
 <\/div>\n
 <\/div>\n

<\/span><\/i><\/b><\/p>\n

Plugins<\/span><\/h2>\n
Sometimes, we\u2019d like to treat a whole application as a black-box framework. We might like a sophisticated end user to be able to extend the application.<\/div>\n
 <\/div>\n
A plugin<\/i> is code, typically developed by a framework user, that provides extra capabilities to an application. It is useful when the application provides a framework (in the broadest sense), but different users might need their own handling of the details.<\/div>\n
 <\/div>\n
Adobe Photoshop\u2122, a top-end graphics tool, is probably the biggest popularizer of the plugin approach. Adobe didn\u2019t want to be responsible for creating and marketing every cool image filter imaginable. Instead, they provide a mechanism where you can buy a plugin from a third party, and install it into Photoshop. The plugin becomes a new tool available to the user, as if it had been built in originally.<\/div>\n
 <\/div>\n
A \u201ctraditional\u201d way to provide such a feature is to provide a pipeline to a separate process. For example, web servers have traditionally used CGI scripts in this way. When a web page is requested that uses a CGI script, the web server creates a process for the script, builds a pipeline to it, sends the script the data from the request, and sends the output of the pipline as the result of the request.<\/div>\n
 <\/div>\n\n\n\n\n\n
\n
Separate process<\/b><\/div>\n<\/td>\n<\/tr>\n
\n
Advantages<\/b><\/div>\n<\/td>\n
\n
Disadvantages<\/b><\/div>\n<\/td>\n<\/tr>\n
\n
\u00b7        <\/span><\/span> Well-defined interface.<\/div>\n
\u00b7        <\/span><\/span> Separate address space prevents a bad script from crashing the whole process.<\/div>\n<\/td>\n
\n
\u00b7        <\/span><\/span> Separate address space requires explicit transfer of information.<\/div>\n
\u00b7        <\/span><\/span> Creating processes adds overhead (mitigated by pooling).<\/div>\n
\u00b7        <\/span><\/span> May require multiple copies of data.<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
 <\/div>\n
An alternative is to use threads, running the script in a separate thread within the server.<\/div>\n
 <\/div>\n
 <\/div>\n\n\n\n\n\n
\n
Separate thread in one process<\/b><\/div>\n<\/td>\n<\/tr>\n
\n
Advantages<\/b><\/div>\n<\/td>\n
\n
Disadvantages<\/b><\/div>\n<\/td>\n<\/tr>\n
\n
\u00b7        <\/span><\/span> Well-defined interface.<\/div>\n
\u00b7        <\/span><\/span> Easier to pass information to the script.<\/div>\n
\u00b7        <\/span><\/span> Less overhead to create thread (rather than process).<\/div>\n<\/td>\n
\n
\u00b7        <\/span><\/span> Trouble in one thread can hurt the whole process.<\/div>\n
\u00b7        <\/span><\/span> The thread has access to the internals of the whole application.<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
 <\/div>\n
In Java, servlets take this approach. (Because Java is a relatively safe language, we run less risk of the script crashing the server than we would for, say, C programs.) Some web servers, such as Apache, have been modified to take this approach for languages other than Java.<\/div>\n
 <\/div>\n
Another alternative would be to provide the whole framework of the application.<\/div>\n
 <\/div>\n\n\n\n\n\n
\n
Make whole framework\/application available<\/b><\/div>\n<\/td>\n<\/tr>\n
\n
Advantages<\/b><\/div>\n<\/td>\n
\n
Disadvantages<\/b><\/div>\n<\/td>\n<\/tr>\n
\n
\u00b7        <\/span><\/span> Allows developer to control all aspects.<\/div>\n
 <\/div>\n<\/td>\n
\n
\u00b7        <\/span><\/span> Requires developer to control all aspects.<\/div>\n
\u00b7        <\/span><\/span> Gives away too much intellectual property.<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
 <\/div>\n
 <\/div>\n
A plugin provides an intermediate point in this design space.<\/div>\n
 <\/div>\n
\u00b7        <\/span><\/span> Only part of the application framework is exposed. The preserves much of the developer\u2019s intellectual property, but still exposes a well-defined piece. (This exposure is the price of letting others extend the application.)<\/div>\n
\u00b7        <\/span><\/span> The plugin code typically runs in the main thread, or a separate thread in the application. It is quick enough to start up that it can be used to extend a menu or palette.<\/div>\n
\u00b7        <\/span><\/span> \u201cIntegration\u201d of the plugin with the application is done at run-time. (The developer has to integrate and test with the application, but to the end user it\u2019s \u201cplug-n-play\u201d.)<\/div>\n
 <\/div>\n
We’ll look at plugins from the perspective of the end user, the plugin developer, and the framework developer. We’ll explore how <\/span> plugins can be implemented in Java. Finally, we’ll add plugin support to the web site manager we developed in a previous chapter.<\/div>\n
 <\/div>\n

<\/span><\/i><\/b><\/p>\n

Plugins as Seen by the End-User<\/span><\/h2>\n
 <\/div>\n
Plugins are simple from the end-user’s point of view. The user has the main application, and decides to add a plugin. Once the plugin is installed, the user has a new feature in the application.<\/div>\n
 <\/div>\n
How does the user install the plugin? Several possibilities are common:<\/div>\n
\u00b7        <\/span><\/span> The main application detects the need for the plugin, and asks permission to <\/span> install it. (Netscape Communicator (TM) does this.)<\/div>\n
\u00b7        <\/span><\/span> The user can instruct the main application to install a new plugin. The main application typically <\/span> uses a file dialog to help the user locate the plugin.<\/div>\n
\u00b7        <\/span><\/span> The plugin may control the process, by having an installation program the user must run. (Then the user is often prompted to locate the main application.)<\/div>\n
\u00b7        <\/span><\/span> Installation is \u201cby convention.\u201d The user may be instructed to drag a file (or files) to a particular directory of the main application.<\/div>\n
 <\/div>\n
Any of these mechanisms are workable. The key is to make it easy for the user.<\/div>\n
 <\/div>\n
 <\/div>\n

Plugins as Seen by the Plugin Developer<\/span><\/h2>\n
The plugin developer typically works with a software development kit (SDK) for the plugin, provided by the developer of the main application.<\/div>\n
 <\/div>\n
The SDK will contain documentation, source code, object code, miscellaneous scripts and tools, and most importantly, examples. (Some SDKs will include tools such as compilers.)<\/div>\n
 <\/div>\n
The plugin developer will need to know the interface to which the plugin code must conform. (In Java, this will often be a Java \u201cinterface\u201d, rather than inheriting from a particular class.) There may be important support classes as well. There may be information passed to<\/i> the plugin that the plugin must understand.<\/div>\n
 <\/div>\n
The first example is the most critical. It’s the equivalent of “Hello, World” in C. It needn’t do much, but when the developer completes the steps, they will have created and installed a plugin from scratch.<\/div>\n
 <\/div>\n
Once completed, the plugin developer will test the plugin, and develop an installation mechanism (if one isn\u2019t provided by the application directly).<\/div>\n
 <\/div>\n
 <\/div>\n
 <\/div>\n

Plugins as Seen by the Framework\/Application Developer<\/span><\/h2>\n
The framework developer must define the interface for the plugin developer. In many ways, this is similar to defining any other framework interface. You want to keep it “narrow” – dependent on as few types as possible. This way, you only expose a limited amount of information to the plugin developer.<\/div>\n
 <\/div>\n
An easy way to allow for plugins is to designate a directory as the target location. This should be in the tree of class files: the plugin mechanism must be able to locate them.<\/div>\n
 <\/div>\n
The framework needs a mechanism for loading the plugin files. (We’ll see the details in the next section.)<\/div>\n
 <\/div>\n
               <\/span>                <\/span> Locate the plugins<\/div>\n
               <\/span>                <\/span> Load their classes<\/div>\n
               <\/span>                <\/span> Instantiate and hook up instances<\/div>\n
 <\/div>\n
Once instantiated, the plugins look like any other class. Usually, they\u2019re added to a special menu or palette, to make them available to the end user at runtime.<\/div>\n
 <\/div>\n
Next, the framework developer must develop documentation and examples, as described in the previous section.<\/div>\n
 <\/div>\n
[TBD: The SDK]<\/div>\n
 <\/div>\n

<\/span><\/p>\n

 <\/div>\n

Plugins in Java<\/span><\/h2>\n
Set directory<\/div>\n
Class.forName(“file”) -> Class<\/div>\n
Class.newInstance()<\/div>\n
 <\/div>\n
Use reflection:<\/div>\n
Class c = Class.forName(“Package.classname”);<\/div>\n
Object o = c.newInstance();<\/div>\n
MyClass c = (MyClass) obj;<\/div>\n
 <\/div>\n
 <\/div>\n
 <\/div>\n

<\/span><\/i><\/b><\/p>\n

The Site Manager as a Plugin<\/span><\/h2>\n
 <\/div>\n
 <\/div>\n
Site manager plugins:<\/div>\n
\u00b7        <\/span><\/span> Look up plugins<\/div>\n
\u00b7        <\/span><\/span> Create main form<\/div>\n
\u00b7        <\/span><\/span> For each plugin, make listener & insert panel<\/div>\n
\u00b7        <\/span><\/span> Select top node<\/div>\n
 <\/div>\n
 <\/div>\n
Make graph:<\/div>\n
 <\/span> getCanReverseLinks()<\/div>\n
to tell if in-links accessible.<\/div>\n
 <\/div>\n
If false, enum always empty (not null).<\/div>\n
\n
 <\/div>\n<\/div>\n
 <\/div>\n
PageSelectionListener<\/div>\n
 <\/div>\n
PageHandler implements PageSelectionListener<\/div>\n
 <\/span> void pageSelected (PageSelectionEvent e) { …}<\/div>\n
 <\/div>\n
 <\/div>\n
Page view Plugin extends PageSelectionListener<\/div>\n
               <\/span> No-arg constructor () required<\/div>\n
               <\/span> void pageSelected (PageSelectionEvent e) { …}<\/div>\n
               <\/span> String getShortName() { …}<\/div>\n
               <\/span> String getLongName() { …}<\/div>\n
               <\/span> JPanel getPanel() { …}<\/div>\n
 <\/div>\n
Or an ObjectSelectionListener?<\/div>\n
 <\/div>\n
PageScorePlugin<\/div>\n
 <\/span> columns<\/div>\n
 <\/span> values<\/div>\n
 <\/span> min? <\/span> max?<\/div>\n
 <\/span> Range limits? (red\/yellow\/green?)<\/div>\n
 <\/span> int valueFor (page)<\/div>\n
 <\/div>\n
 <\/div>\n

<\/span><\/i><\/b><\/p>\n

Plugin<\/span><\/h2>\n
 <\/div>\n
Root directory<\/div>\n
Directory to search<\/div>\n
Class to match<\/div>\n
Return array of all classes in the list that match the class<\/div>\n
 <\/div>\n
 <\/div>\n
DefaultMutableTreeNode<\/div>\n
 <\/div>\n
public class FileTree extends DefaultTreeModel {<\/div>\n
  <\/span> public FileTree(File base) {<\/div>\n
               <\/span> super(new FileTreeNode(base));<\/div>\n
 <\/span> }<\/div>\n
}<\/div>\n
 <\/div>\n
public class FileTreeNode extends DefaultMutableTreeNode {<\/div>\n
               <\/span> public FileTreeNode(File base) { super(base); base.isDirectory();}<\/div>\n
               <\/span> protected boolean loaded;<\/div>\n
               <\/span> public int getChildCount() {<\/div>\n
               <\/span>                <\/span> if (!loaded) {<\/div>\n
                               <\/span>                <\/span> File thisFile = (File)userObject;<\/div>\n
                               <\/span>                <\/span> String files[] = thisFile.list();<\/div>\n
                               <\/span>                <\/span> for (int i = 0; i < files.length; i++) {<\/div>\n
                               <\/span>                <\/span>                <\/span> add(new FileTreeNode(new File(thisFile, files[i])));<\/div>\n
                               <\/span>                <\/span> }<\/div>\n
                               <\/span>                <\/span> loaded = true;<\/div>\n
               <\/span>                <\/span> }<\/div>\n
               <\/span>                <\/span> return super.getChildCount();<\/div>\n
               <\/span> }<\/div>\n
 <\/div>\n
Note:<\/div>\n
\u00b7        <\/span><\/span> File.toString() is full path<\/div>\n
\u00b7        <\/span><\/span> Not alphabetical<\/div>\n
\u00b7        <\/span><\/span> Method to select row given file<\/div>\n
 <\/div>\n

<\/span><\/p>\n

public class GraphBrowser extends JPanel <\/span> implements GraphListener {<\/div>\n
               <\/span> GraphBrowser(Graph)<\/div>\n
               <\/span> GraphBrowser(Graph, Node)<\/div>\n
               <\/span> GraphBrowser()  <\/span>                <\/span> –??<\/div>\n
               <\/span> addObjectSelectionListener (ObjectSelectionListener osl) { …}<\/div>\n
               <\/span> removeObjectSelectionListener(ObjectSelectionListener osl) { …}<\/div>\n
               <\/span> setGraph(Graph)<\/div>\n
               <\/span> setNode(Node)   <\/span>                <\/span> \/\/ may be null<\/div>\n
}<\/div>\n
 <\/div>\n
 <\/div>\n
UI:<\/div>\n
               <\/span> List               <\/span> Label=current  <\/span> List<\/div>\n
 <\/div>\n
 <\/div>\n

<\/span><\/i><\/b><\/p>\n

Graph Browser<\/span><\/h2>\n
 <\/div>\n
Graph graph = null<\/div>\n
Node current = null<\/div>\n
 <\/div>\n
JList inList = new JList();   <\/span> \/\/ single-sel<\/div>\n
JList outList = new JList();               <\/span> \/\/ single-sel<\/div>\n
JLabel currentNode = new JLabel();<\/div>\n
 <\/div>\n
GraphBrowser() {<\/div>\n
               <\/span> setLayout(new GridLayout(1,3));<\/div>\n
               <\/span> add(new JScrollPane(inList));<\/div>\n
               <\/span> add(currentNode);<\/div>\n
               <\/span> add(new JScrollPane(outList));<\/div>\n
}<\/div>\n
 <\/div>\n
GraphBrowser(Graph) { …}<\/div>\n
GraphBrowser(Graph, Node) { …}<\/div>\n
 <\/div>\n
void setGraph (Graph g) { graph = g; setNode(null);}<\/div>\n
 <\/div>\n
void setNode (Node n) {<\/div>\n
               <\/span> if (current != n) {<\/div>\n
               <\/span>                <\/span> inList.setModel(new listModel w\/in-nodes for enum);<\/div>\n
               <\/span>                <\/span> Object o = n.getUserObject();<\/div>\n
               <\/span>                <\/span> currentNode.setText(n==null?””:n.toString());<\/div>\n
               <\/span>                <\/span> outList.setModel(new model w\/out-nodes);<\/div>\n
               <\/span>                <\/span> fireSelectionChanged();<\/div>\n
               <\/span> }<\/div>\n
}<\/div>\n
 <\/div>\n
Listeners must make the selection be the new node.<\/div>\n
addObjectSelectionListener (osl o) { …}<\/div>\n
removeObjectSelectionListener(osl O) { …}<\/div>\n
 <\/div>\n

<\/span><\/p>\n

NodeToFileTreeAdapter extends objectSelectionListener {<\/div>\n
               <\/span> public void objectSelected (ObjectSelectionEvent e) {<\/div>\n
               <\/span>                <\/span> Node n = (Node)e.getSelection();<\/div>\n
               <\/span>                <\/span> Page p = (Page)n.getUserObject();<\/div>\n
               <\/span>                <\/span> File f = p.getFile();<\/div>\n
               <\/span>                <\/span> find f in tree<\/div>\n
               <\/span>                <\/span> Set selector to that<\/div>\n
               <\/span> }<\/div>\n
}<\/div>\n
 <\/div>\n

<\/span><\/i><\/b><\/p>\n

PLUGINS – NETSCAPE<\/span><\/h2>\n
 <\/div>\n
http:\/\/developer.netscape.com\/docs\/manuals\/communicator\/plugin\/index.html<\/div>\n
 <\/div>\n
Checks directory<\/div>\n
Registers<\/div>\n
Queries to see if they can handle it<\/div>\n
 <\/div>\n
Check type<\/div>\n
Load plugin<\/div>\n
Initialize<\/div>\n
Create instance<\/div>\n
 <\/div>\n
Plug-in methods: Functions you implement<\/div>\n
Netscape methods: In Communicator, plugin calls them<\/div>\n
Data structures<\/div>\n
 <\/div>\n
 <\/div>\n
When does it apply?<\/div>\n
Beans as programs<\/div>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

What   Why                 vs external program                 vs source code   Mechanics of plugin client                 Code to interface                 Move class files to plugin directory                 Watch it work   Mechanics of server                 Define interface                 Define directory                 Load classes                 Instantiate & call   Plugins in Java   Consequences                 […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"nf_dc_page":"","om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[3],"tags":[],"class_list":["post-16","post","type-post","status-publish","format-standard","hentry","category-frameworks"],"yoast_head":"\nGrowing Frameworks: Plugins - Bill Wake's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"http:\/\/billwake.com\/growing-frameworks-plugins\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Growing Frameworks: Plugins - Bill Wake's Blog\" \/>\n<meta property=\"og:description\" content=\"What   Why                 vs external program                 vs source code   Mechanics of plugin client                 Code to interface                 Move class files to plugin directory                 Watch it work   Mechanics of server                 Define interface                 Define directory                 Load classes                 Instantiate & call   Plugins in Java   Consequences                 […]\" \/>\n<meta property=\"og:url\" content=\"http:\/\/billwake.com\/growing-frameworks-plugins\/\" \/>\n<meta property=\"og:site_name\" content=\"Bill Wake's Blog\" \/>\n<meta property=\"article:published_time\" content=\"1999-06-06T04:08:00+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2020-08-01T20:55:02+00:00\" \/>\n<meta name=\"author\" content=\"Bill Wake\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Bill Wake\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"16 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"http:\\\/\\\/billwake.com\\\/growing-frameworks-plugins\\\/#article\",\"isPartOf\":{\"@id\":\"http:\\\/\\\/billwake.com\\\/growing-frameworks-plugins\\\/\"},\"author\":{\"name\":\"Bill Wake\",\"@id\":\"https:\\\/\\\/billwake.com\\\/#\\\/schema\\\/person\\\/b551cc572ce5d025b081e031a73eec0c\"},\"headline\":\"Growing Frameworks: Plugins\",\"datePublished\":\"1999-06-06T04:08:00+00:00\",\"dateModified\":\"2020-08-01T20:55:02+00:00\",\"mainEntityOfPage\":{\"@id\":\"http:\\\/\\\/billwake.com\\\/growing-frameworks-plugins\\\/\"},\"wordCount\":3257,\"commentCount\":0,\"articleSection\":[\"frameworks\"],\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"CommentAction\",\"name\":\"Comment\",\"target\":[\"http:\\\/\\\/billwake.com\\\/growing-frameworks-plugins\\\/#respond\"]}]},{\"@type\":\"WebPage\",\"@id\":\"http:\\\/\\\/billwake.com\\\/growing-frameworks-plugins\\\/\",\"url\":\"http:\\\/\\\/billwake.com\\\/growing-frameworks-plugins\\\/\",\"name\":\"Growing Frameworks: Plugins - 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