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Netscape Plugin Application Programming Interface (NPAPI) is an application programming interface (API) that allows browser plugins to be developed. It was first developed for Netscape browsers, starting in 1995 with Netscape Navigator 2.0, but was subsequently adopted by other browsers. With the advent of HTML5, all major web browsers have removed support for this API for security reasons.
In NPAPI architecture, a plugin declares content types (e.g. "audio/mp3") that it can handle. When the browser encounters a content type it cannot handle natively, it loads the appropriate plugin, sets aside space within the browser context for the plugin to render and then streams data to it. The plugin is responsible for rendering the data. The plugin runs in-place within the page, as opposed to older browsers that had to launch an external application to handle unknown content types.
NPAPI requires each plugin to implement and expose approximately 15 functions for initializing, creating, deleting and positioning plugin content. NPAPI also supports scripting, printing, full-screen plugins, windowless plugins and content streaming.
LiveConnect was used in Netscape 4 to implement scriptability of NPAPI plugins.
The Open Java Interface-dependent implementation of LiveConnect was removed from the Mozilla source code tree in late June 2009 as part of the Mozilla 2 cleanup effort. It is no longer needed with the release of a redesigned Java Runtime Environment from Sun Microsystems. However the old implementation was restored for Gecko 1.9.2, as Apple had yet to port the newer JRE over to Mac OS X.
The disadvantage of LiveConnect is that it is heavily tied to the version of Java embedded within the Netscape browser. This prevented the browser from using other Java runtimes, and added bloat to the browser download size since it required Java to script plugins. Additionally, LiveConnect is tricky to program: The developer has to define a Java class for the plugin, run it through a specialized Java header compiler and implement the native methods. Handling strings, exceptions and other Java objects from C++ is non-obvious. In addition, LiveConnect uses an earlier and now obsolete application programming interface (API) for invoking native C++ calls from Java, called JRI. The JRI technology has long since been supplanted by JNI.
Full privileges are only granted by default to chrome scripts, i.e. scripts that are part of the application or of an extension. For remote HTML/XHTML/XUL documents, most XPCOM objects are not accessible by the scripts as they have limited privileges due to security reasons. Even if they are accessible (e.g. the XMLHttpRequest object), the usual security restrictions can also be found (e.g. cannot open URLs of other domains).
Mozilla was already using XPCOM to define the interfaces to many objects implemented in C++. Each interface was defined by an IDL file, and run through an IDL compiler that produced header files and a language-neutral type library that was a binary representation of the interface. This binary described the interface, the methods, the parameters, the data structures and enumerations.
XPConnect has no Java dependency. However, the technology is based on XPCOM. Thus the plugin developer must be familiar with reference counting, interfaces and IDL to implement scripting. The dependency on XPCOM led to certain dynamic linking issues (e.g. the fragile base class problem) which had to be solved before the plugin would work correctly with different browsers. XPCOM has since been changed to supply a statically linked version to address such issues. This approach also requires an .xpt file to be installed next to the dynamic-link library (DLL); otherwise the plugin appears to work, but the scripting does not, causing confusion.
At the end of 2004, all major browser companies using NPAPI agreed on NPRuntime as an extension to the original NPAPI to supply scripting, via an API that is similar in style to the old C-style NPAPI and is independent of other browser technologies like Java or XPCOM. It is only supported by Firefox ESR (Extended Support Release) and Safari.
- Internet Explorer versions 3 through 5.5 SP2 supported NPAPI, allowing plugins that functioned in Netscape Navigator to function in Internet Explorer. Support came via a small ActiveX control (named "plugin.ocx") that acted as a shim between ActiveX and the NPAPI plugin. Microsoft dropped support in version 5.5 SP2 onwards for security reasons.
- Google Chrome 45, in September 2015, permanently dropped all NPAPI support from all platforms. In September 2013, Google announced that it would phase out NPAPI support in its Google Chrome browser during 2014, stating that "[its] 90s-era architecture has become a leading cause of hangs, crashes, security incidents, and code complexity". In May 2014, NPAPI support was removed from the Linux version of Chrome 35 and later. In April 2015, Chrome for Windows and OS X (versions 42 and later) disabled NPAPI support by default. However, until September 2015, users could re-enable NPAPI.
- Opera dropped support with version 37 in May 2016.
- Firefox release 52.0 in March 2017 removed all support for NPAPI except for Flash. Meanwhile, the ESR channel retained general support for this feature with version 52esr being the last NPAPI resort. Firefox 69.0 will disable the Flash NPAPI by default. 
- Safari has dropped support for all NPAPI plugins except for Flash with version 12 released in September 2018.
The following list of web browsers support all NPAPI plugins:
- 360 Secure Browser (Chinese: 360安全浏览器) or 360 Extreme Explorer (Chinese: 360极速浏览器)
- CometBird
- GNOME Web
- Konqueror
- K-Meleon (Goanna engine)
- Maxthon
- Midori
- Pale Moon (Pale Moon future roadmap)
- UC Browser
- Waterfox (Allowing running of all 64-bit NPAPI plugins)
Internet Explorer and browsers based on Internet Explorer use ActiveX controls, ActiveX documents and ActiveX scripting to offer in-page extensibility on par with NPAPI. Although commonly associated with Internet Explorer, ActiveX is integration technology that allows any computer program to integrate parts of other computer programs that support such integration. Internet Explorer, however, is discontinued and its replacement, Microsoft Edge, does not support ActiveX.
On 12 August 2009 a page on Google Code introduced a new project called Pepper, with the associated Pepper Plugin API (PPAPI); PPAPI is a derivative of NPAPI aimed to make plugins more portable and more secure. This extension is designed specifically to ease the implementation of out-of-process plugin execution.
In February 2012 Adobe Systems announced that future Linux versions of Adobe Flash Player would be provided only via PPAPI. The previous release, Flash Player 11.2, with NPAPI support, would receive security updates for five years. In August 2016 Adobe announced that, contrary to their previous statement, it would again support the NPAPI Flash Player on Linux and keep releasing new versions of it.
- For technical details, see the Mozilla Developer Documentation on LiveConnect.
- "442399 – remove LiveConnect from the tree". mozilla.org.
- "517355 – Restore OJI, Liveconnect and the JEP on the 1.9.2 branch on OS X". mozilla.org.
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- ""Change in support for Acrobat and Reader plug-ins in modern web browsers"". Adobe. 8 February 2016.
- "Oracle deprecates the Java browser plugin, prepares for its demise". Ars Technica. 28 January 2016. Retrieved 15 April 2016.
- "Netscape-Style Plug-ins Do Not Work After Upgrading Internet Explorer". Support (3.3 ed.). Microsoft. 27 July 2007.
- Giannandrea, J. (4 September 2001). "Microsoft breaks Web Plugins in Windows XP". meer.net. Archived from the original on 16 October 2007.
- "Description of Internet Explorer Support for Netscape-Style Plug-ins". Support (3.4 ed.). Microsoft. 31 January 2007.
- "Microsoft Security Bulletin MS03-015 – Critical". Security TechCenter. Microsoft. 23 April 2003.
- "The final countdown for NPAPI". Chromium Blog.
- Google will start blocking most Netscape Plug-In API plug-ins in January 2014, will whitelist Silverlight, Unity & others". TechCrunch. 23 September 2013.
- "Google looks to drop Netscape Plugin API support in Chrome, starting with blocking most plugins in January 2014". The Next Web. 23 September 2013.
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- "Release Notes for Firefox 52.0".
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- "Why do Java, Silverlight, Adobe Acrobat and other plugins no longer work?". Mozilla support. Archived from the original on 2017-03-07. Retrieved 2017-03-06.
- "1519434 - Disable Flash support by default in Firefox 69". bugzilla.mozilla.org. Retrieved 2019-01-14.
- "Firefox 69: Flash disabled by default - gHacks Tech News". www.ghacks.net. Retrieved 2019-01-14.
- Clover, Juli (September 17, 2018). "Apple Releases Safari 12 for macOS Sierra and macOS High Sierra". MacRumors.
- "NPAPI 是重型武器，当别的方法无法到达你的目的时，才建议使用。". NPAPI 插件.
- "Support for all NPAPI plugins (Unity, Silverlight, Flash, Java, authentication plugins, etc.)". Basilisk features.
- "NPAPI, including Adobe Flash and IcedTea plug-ins support". Archived from the original on 2016-05-12.
- "Most third-party plug-ins designed for Mozilla/Netscape will also work with K-Meleon".Third party plugins.
- "Load all NPAPI-Plugins, Java, Silverlight, etc." K-Meleon Hybrid (Goanna over Pro).
- "Full and ongoing support for NPAPI plugins (Java, Silverlight, etc.)". Pale Moon: Technical Details
- "Pale Moon supports NPAPI plug-ins. Unlike Firefox, we will not be deprecating or removing support for these kinds of plug-ins". Pale Moon future roadmap.
- "32-bit plugin using carbon-based NPAPI". Release Notes.
- "After the JRE is installed on your machine, copy NPOJI610.dll from the install directory to your SeaMonkey plugins directory". Java, Plug-ins and Other Extras.
- "We use the NPAPI plugin architecture (just like Mozilla) so just install the plugins normally, and things should work". Uzbl FAQ.
- "They should be used responsibly, but Waterfox still supports the use of Java and Silverlight plugins, as well as any other 64-Bit NPAPI plugins. Support for NPAPI Plugins.
- "Description of ActiveX Technologies". Support. Microsoft. 19 January 2007.
- "ppapi". Google Code. Google. Archived from the original on 2010-07-02.
- "Getting Started: Background and Basics – The Chromium Projects". chromium.org.
- "Concepts - ppapi - Important concepts for working with PPAPI. - Pepper Plugin API – Google Project Hosting". google.com.
- "Adobe and Google Partnering for Flash Player on Linux". adobe.com.
- Campbell, Chris (31 August 2016). "Beta News – Flash Player NPAPI for Linux". Adobe AIR and Adobe Flash Player Team Blog. Adobe Systems. Retrieved 8 September 2016.
- Plugin development documentation on Mozilla Developer Center, including the NPAPI API
- An ActiveX control that hosts plugins – a replacement for plugin.ocx that was removed from Internet Explorer.
- Book on Programming Netscape Plug-Ins by Zan Oliphant
- Nixysa: A glue code generation framework for NPAPI plugins. Apache 2.0 license.
- NPAPI Tutorial Building a Firefox Plugin (Part two, Part three, Part four)
- Opera 15+ extensions documentation