20.17

java.lang.SecurityManager

THE PACKAGE JAVA.LANG

public void checkAccept(String host, int port)

throws SecurityException;

public void checkSetFactory()

throws SecurityException;

public boolean checkTopLevelWindow()

throws SecurityException;

public void checkPackageAccess(String packageName)

throws SecurityException;

public void checkPackageDefinition(String packageName)

throws SecurityException;

}

A running Java program may have a security manager, which is an instance of

class

SecurityManager

. The current security manager is the one returned by the

method invocation

System.getSecurityManager()

 ( 20.18.4).

The

SecurityManager

 class contains a large number of methods whose

names begin with  

check

 . They are called by various methods throughout the

Java libraries before those methods perform certain sensitive operations. The

invocation of such a check method typically looks like this:

SecurityManager security = System.getSecurityManager();

if (security != null) {

security.checkXXX(arguments);

}

The security manager is thereby given an opportunity to prevent completion

of the operation by throwing an exception. The usual convention is that a security

manager checking routine simply returns if the operation is permitted, or throws a

SecurityException

 if the operation is not permitted. In one case, namely

checkTopLevelWindow

 ( 20.17.27), the checking routine must return a

boolean

value to indicate one of two levels of permission.

20.17.1

protected boolean inCheck = false;

By convention, this field should be assigned the value

true

 whenever a security

check is in progress. This matters when one of the checking routines needs to call

outside code to do its work. Outside code can then use the method

getInCheck

( 20.17.9) to test the status of this flag.

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