java.endorsed.dirs 和 java.ext.dirs 系统属性说明

JVM系统属性中有两个特殊的属性：

{java.ext.dirs} 可选包扩展机制
{java.endorsed.dirs} 包升级替换机制

相关资料：

http://docs.oracle.com/javase/1.4.2/docs/guide/extensions/spec.html
http://docs.oracle.com/javase/6/docs/technotes/guides/standards/index.html

JVM Endorsed Standards Override Mechanism

Goal

Provide a mechanism to override certain classes included in J2SE platform.

 

Reason

Those endorsed standards are defined through processes other than JCP. In other words these standards are not under the control of JCP, such as w3c and OMG.

So these standards may subject to change between releases of Java2 platform. Such a mechanism can let developers and software vendors to take advantage of

new revisions to endorsed standards.

 

Means

If JVM can find certain classes in a special directory called endorsed dir, these classes definition will override those included in J2SE platform. The

default endorsed dir locates in \lib\endorsed [Microsoft Windows] or /lib/endorsed [Solaris or Linux]

Here refers to the directory where the runtime software is installed (which is the top-level directory of the Java 2 Runtime Environment or the

jre directory in the Java 2 SDK).

You can change the default location to another locations, however, by setting a JVM option called java.endorsed.dirs. Separate multiple directories with

File.pathSeparatorChar.

 

Scope

Obviously, not all platform classes can be overridden. Classes can be overridden are listed on the page of

http://java.sun.com/j2se/1.4.2/docs/guide/standards/.