Table of Contents
This chapter describes servlet auto reloading that helps reduce the web application development time.
In general, the Java EE development life cycle consists of the following steps.
Edit
Build
Deploy
Test
Servlet classes are often modified when developing Java EE applications, especially web applications. Much effort has been made to shorten the development process, and WebLogic 10.3 provides functions that reduce the redeployment process through FastSwap.
Java EE 5 introduced a function that can redefine classes without terminating the class loader or instances while applications are running, but declared fields and methods cannot be modified.
Up to JEUS 6, to reload a modified class, the application must be redeployed or a new class loader must be created using automatic reloading to replace the existing class loader. Automatic reloading can be executed at a fixed interval or on demand.
However, if the application is too big, it takes a long time to redeploy the application and automatic reloading uses as much overhead as redeployment because it recreates the class loader.
JEUS 7 provides a function that can reload modified Java classes dynamically without reloading them(Automatic reloading) by using the JDK instrumentation package. This function is called JEUS HotSwap. It can currently only be used for web application classes.
The automatic reloading (JEUS HotSwap activated/inactivated)function can generate unexpected system overhead in a production environment, so it is recommended to only use it in a development environment.
This section describes how to configure and operate automatic reloading.
JEUS must be configured to use HotSwap before it is started. Auto reloading that does not have the JEUS HotSwap function starts without the system option when the server is started.
The system option is jeus.server.useHotSwapAgent and its default value is 'false'. If it is not set, auto reloading cannot be used.
The following are examples of configuring JEUS 7 start scripts.
startDomainAdminServer script configuration
[Example 9.1] Configuring Automatic Reloading: <<startDomainAdminServer>>
...
"${JAVA_HOME}/bin/java" $VM_OPTION $SESSION_MEM
-Xbootclasspath/p:"${JEUS_HOME}/lib/system/extension.jar"
...
-Djeus.server.useHotSwapAgent=true
...
jeus.launcher.Launcher ${BOOT_PARAMETER}
...
startManagedServer script configuration
[Example 9.2] Configuring Automatic Reloading: <<startManagedServer>>
...
"${JAVA_HOME}/bin/java" $VM_OPTION $SESSION_MEM
-Xbootclasspath/p:"${JEUS_HOME}/lib/system/extension.jar"
...
-Djeus.server.useHotSwapAgent=true
...
jeus.launcher.ManagedServerLauncher ${BOOT_PARAMETER}
...
Enable automatic reloading during development, but disable it in the production environments.
To use automatic reloading, <auto-reload> must be configured in jeus-web-dd.xml. To use JEUS HotSwap, <use-jvm-hotswap> inside <auto-reload> must be set to true.
These functions can only be used in class files that are packaged as directories(exploded directories) of web applications. These functions are used to dynamically reload modified classes in already deployed directories. In other words, they can only be used for modified classes in the 'WEB-INF/classes' directory.
Configure <auto-reload> in jeus-web-dd.xml as in the following. Set the monitoring interval for auto reloading in WebAdmin. Refer to "1.6.2. Monitoring" for more information about setting the monitoring interval in WebAdmin.
[Example 9.3] Configuring Automatic Reloading: <<jeus-web-dd.xml>>
<jeus-web-dd xmlns="http://www.tmaxsoft.com/xml/ns/jeus" version="7.0">
. . .
<auto-reload>
<enable-reload>true</enable-reload>
<use-jvm-hotswap>true</use-jvm-hotswap>
<check-ok-demand>true</check-ok-demand>
</auto-reload>
. . .
</jeus-web-dd>
Refer to "9.2.3. Configuration Results" for more information about each tag.
In JEUS, automatic reloading is performed when a class is modified after the application has been deployed.
Automatic reloading uses the following settings.
| Tag | Description |
|---|---|
| <enable-reload> | Option to use automatic reloading. Must be set to true. |
| <use-jvm-hotswap> |
|
| <check-on-demand> |
|
JEUS HotSwap supports POJOs and web application classes in exploded directories.
The following modifications are supported by JEUS HotSwap.
Adding and deleting static class constructors
Adding and deleting regular class constructors
Changing static method bodies
Changing regular method bodies
The following list of class modifications are supported by JEUS HotSwap.
Java class instances (non-abstract)
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting methods | No | |
| (1) Adding and deleting fields | No | |
| (2) Changing method bodies | Yes | |
| (3) Adding and deleting constructors | Yes | |
| (4) Changing field modifiers | No |
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting methods | No | |
| Changing method bodies | Yes |
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting abstract methods | No | |
| Change types (1)-(4) of Java class instances | Yes |
| Java Change Type | Supported | Notes |
|---|---|---|
| Change types (1)-(4) of Java class instances | Yes |
| Java Change Type | Supported | Notes |
|---|---|---|
| Change types (1)-(4) of Java class instances | Yes |
| Java Change Type | Supported | Notes |
|---|---|---|
| Change types (1)-(4) of Java class instances | Yes |
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting variables | No | |
| Adding and deleting methods | No |
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting fields | Invalid | Java does not support this. |
| Adding and deleting methods | No |
| Java Change Type | Supported | Notes |
|---|---|---|
| Change types (1)-(4) of Java class instances | Yes |
| Java Change Type | Supported | Notes |
|---|---|---|
| Change types (1)-(4) of Java class instances | Yes |
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding methods | No |
| Java Change Type | Supported | Notes |
|---|---|---|
| Accessing existing fields and methods | Yes | |
| Accessing new methods | No | New methods are not visible when using reflection. |
| Accessing new fields | No | New fields are not visible when using reflection. |
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting class annotations | No |
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting method and property annotations | No |
| Java Change Type | Supported | Notes |
|---|---|---|
| Change types (1)-(4) of Java class instances | Yes |
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting methods | No | Changing EJB interfaces related to reflection is not supported. |
EJB 3.0 Session and MDB implementation classes
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting methods and fields | No | Only supported classes among the classes referenced by EJB can be modified. |
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting methods and fields | No | Only supported classes among the classes referenced by EJB can be modified. |
| Java Change Type | Supported | Notes |
|---|---|---|
| Adding and deleting methods and fields | No | Only supported classes among the classes referenced by EJB can be modified. |
Not all types of changes are supported by JEUS HotSwap. If an attempt is made to make an unsupported change, the JDK will return an UnsupportedOperationException. JEUS receives the error message and logs it as "Re-transforming all modified classes in the servlet context [AAA] failed." When this happens,the application will be not updated dynamically, but instead, an automatic reloading will be executed.
The following changes are not supported in JEUS HotSwap.
Since Java reflection results cannot include newly modified fields and methods, using reflection on modified classes can cause unexpected results.
Changing the existing class hierarchy is not supported.
For example, changing a list that implements class interfaces or modifying the class' superclass are not supported.
Adding or deleting Java annotations are is supported, due to the limitations of reflection.
Adding or deleting EJB interface methods is not supported because it requires the reflection of the changes.
Adding or deleting enumeration constants is not supported.
Adding or deleting finalize methods is not supported.