Modifying the Unmodifiable Map

Some days back I needed to work on setting up the system environment variables. Well, actually the need was to add a new env variable to the system properties.

In Java how this works is, you can get all the system variables values using System.getenv(). This function returns a Map, key-value pair. But, this is read only. You cannot modify the map, say, to put in an extra env variable into the system properties.

If you try, you will get:

java.lang.IllegalAccessException: Class org.system.SystemTest can not access a member of class java.util.Collections$UnmodifiableMap with modifiers "private final"

at sun.reflect.Reflection.ensureMemberAccess(Unknown Source)

at java.lang.reflect.Field.doSecurityCheck(Unknown Source)

at java.lang.reflect.Field.getFieldAccessor(Unknown Source)

at java.lang.reflect.Field.get(Unknown Source)

at org.system.SystemTest1.main(SystemTest1.java:34)

So basically, the modifier of java.util.Collections$UnmodifiableMap class is set to false. This will not let you override the existing environment variables.

After going through lot of sites and scavenging the forums I came across the following "cool" code.

Class[] classes = Collections.class.getDeclaredClasses();

for(Class cl : classes) {

if("java.util.Collections$UnmodifiableMap".equals(cl.getName())) {

Field field = cl.getDeclaredField("m");

field.setAccessible(true);

Object obj = field.get(env);

Map map = (Map) obj;

map.put("NEW_PROPERTY", "NEW_PROPERTY_VALUE");

map.putAll(env);

}

}

This code changes the modifier field "m" to true and that will give you the liberty to change the unmodifiable map :)

Output:

Before:

(OS,Windows_NT)

(PROCESSOR_LEVEL,15)

(ANT_HOME,C:\apache-ant-1.7.1)

(windir,C:\WINDOWS)

(SystemRoot,C:\WINDOWS)

(asl.log,Destination=file;OnFirstLog=command,environment)

(NUMBER_OF_PROCESSORS,1)

After:

(OS,Windows_NT)

(PROCESSOR_LEVEL,15)

(ANT_HOME,C:\apache-ant-1.7.1)

(windir,C:\WINDOWS)

(SystemRoot,C:\WINDOWS)

(asl.log,Destination=file;OnFirstLog=command,environment)

(NUMBER_OF_PROCESSORS,1)

(NEW_PROPERTY,NEW_PROPERTY_VALUE)