Version: 9.4.20.v20190813 |

Quickstart Webapps
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## Quickstart Webapps

The auto discovery features of the Servlet specification can make deployments slow and uncertain. Auto discovery of Web Application configuration can be useful during the development of a webapp as it allows new features and frameworks to be enabled simply by dropping in a jar file. However, for deployment, the need to scan the contents of many jars can have a significant impact of the start time of a webapp.

With the release of Jetty 9.2, a quickstart module was included which allows a webapp to be pre-scanned and preconfigured. This means that all the scanning is done prior to deployment and all configuration is encoded into an effective `web.xml`, called `WEB-INF/quickstart-web.xml`, which can be inspected to understand what will be deployed before deploying. Not only does the `quickstart-web.xml` contain all the discovered Servlets, Filters and Constraints, but it also encodes as context parameters all discovered:

- ServletContainerInitializers
- HandlesTypes classes
- Taglib Descriptors

With the quickstart mechanism, Jetty is able to entirely bypass all scanning and discovery modes and start a webapp in a predictable and fast way. Tests have shown that webapps that took many seconds to scan and deploy can now be deployed in a few hundred milliseconds.

### Setting up Quickstart

#### Prerequisites

##### Jetty Distribution

In a standard Jetty distribution the quickstart module can be configured with the following command:

    $ java -jar $JETTY_HOME/start.jar --add-to-start=quickstart

##### Embedded

In a Maven project you add a dependency on the artifact `jetty-quickstart`.

    <dependency>
        <groupId>org.eclipse.jetty</groupId>
        <artifactId>jetty-quickstart</artifactId>
        <version>9.4.20.v20190813</version>
    </dependency>

#### Configuration

Webapps need to be instances of `org.eclipse.jetty.quickstart.QuickStartWebApp` rather than the normal `org.eclipse.jetty.webapp.WebAppContext`.

`org.eclipse.jetty.quickstart.QuickStartWebApp` instances offer the same setters as the familiar `org.eclipse.jetty.webapp.WebAppContext`, with the addition of:

autoPreconfigure
  (true/false). If true, the first time the webapp is run, the WEB-INF/quickstart-web.xml is generated BEFORE the webapp is deployed. Subsequent runs use the previously generated quickstart file.
originAttribute
  The name of an attribute to insert into the generated elements in quickstart-web.xml that gives the origin of the element. By default it is `origin`.
generateOrigin
  (true/false). By default it is `false`. If true, the origin attribute will be inserted into each element in quickstart-web.xml. Note that origin attributes will also be generated if debug log level is enabled.

> ### Note

> If you are using Spring-Boot you must set `generateOrigin` to true.

The origin is either a descriptor eg web.xml,web-fragment.xml,override-web.xml file, or an annotation eg @WebServlet. For xml validation each attribute must be unique, and therefore an integer counter is appended to each value. Some examples of elements with origin attribute information are:

    <listener origin="DefaultsDescriptor(file:///path/to/distro/etc/webdefault.xml):21">
    <listener origin="WebDescriptor(file:///path/to/base/webapps/test-spec/WEB-INF/web.xml):22">
    <servlet-class origin="FragmentDescriptor(jar:file:///path/to/base/webapps/test-spec/WEB-INF/lib/test-web-fragment.jar!/META-INF/web-fragment.xml):23">
    <servlet-class origin="@WebServlet(com.acme.test.TestServlet):24">

##### In XML

If a web application already has a context xml file, eg `webapps/myapp.xml` file, simply change the class in the `Configure` element. Otherwise, create a context xml file with the following information (in addition to the usual setting of contextPath, war etc):

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "http://www.eclipse.org/jetty/configure_9_3.dtd">
    <Configure class="org.eclipse.jetty.quickstart.QuickStartWebApp">
      <Set name="autoPreconfigure">true</Set>
    </Configure>

##### In Code

Create an instance of `org.eclipse.jetty.quickstart.QuickStartWebApp` rather than the normal `org.eclipse.jetty.webapp.WebAppContext`. You then use the QuickStartWebApp instance in exactly the same way that you would a WebAppContext.

Here’s a snippet:

        QuickStartWebApp webapp = new QuickStartWebApp();
        webapp.setAutoPreconfigure(true);

##### Pre-generating the quickstart-web.xml file

Rather than use the `autoPreconfigure` feature of the QuickStartWebApp - which lazily generates the `quickstart-web.xml` file - you can eagerly pre-generate it for an existing war by invoking as a main class `org.eclipse.jetty.quickstart.PreconfigureQuickStartWar`. Note that you will need to provide all necessary jetty jars on the command line classpath. This will unpack the war if necessary, and create the `quickstart-web.xml` before the first deployment:

    $ java -cp [jetty classpath] org.eclipse.jetty.quickstart.PreconfigureQuickStartWar myapp.war

Run the class with no arguments to see other runtime options.

Alternatively, you could use the Jetty Maven Plugin goal `jetty:effective-web-xml`: this will generate quickstart information, but print it to stderr. The goal provides a configuration option to save the output to a file, which you can then copy into your webapp’s WEB-INF dir. Note that as the Jetty Maven Plugin is a general tool for running webapps, it may have more jars on its classpath than are needed by your application, and thus may generate extra quickstart information: we recommend that you use this goal only as a quick guide to the type of information that quickstart generates.

### Avoiding TLD Scans with precompiled JSPs

Of course precompiling JSPs is an excellent way to improve the start time of a web application. As of Jetty 9.2 the Apache Jasper JSP implementation has been used and has been augmented to allow the TLD scan to be skipped. This can be done by adding a `context-param` to the `web.xml` file (this is done automatically by the Jetty Maven JSPC plugin):

    <context-param>
      <param-name>org.eclipse.jetty.jsp.precompiled</param-name>
      <param-value>true</param-value>
    </context-param>

### Bypassing start.jar

The Jetty `start.jar` mechanism is a very powerful and flexible mechanism for constructing a `classpath` and executing a configuration encoded in Jetty XML format. However, this mechanism does take some time to build the `classpath`. The start.jar mechanism can be bypassed by using the `–dry-run` option to generate and reuse a complete command line to start Jetty at a later time:

    $ RUN=$(java -jar $JETTY_HOME/start.jar --dry-run)
    $ eval $RUN

Note that `--dry-run` may create a properties file in the temp directory and include it on the generated command line. If so, then a copy of the temporary properties file should be taken and the command line updated with it’s new persistent location.

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