e212. enabling the security manager.txt

这里面包含了一百多个JAVA源文件

By default, no security manager is enabled which means that all security checks to protected resources and operations are disabled. To enable security checks, the security manager must be enabled. 
Once enabled, policy files determine the type of access an entity has on a resource. For more details, see e220 Managing Policy Files. 

This example enables the security manager. 

    // Before the security manager is enabled, this call is possible
    System.setProperty("java.version", "malicious data");
    
    try {
        // Enable the security manager
        SecurityManager sm = new SecurityManager();
        System.setSecurityManager(sm);
    } catch (SecurityException se) {
        // SecurityManager already set
    }
    
    // This call is no longer possible; an AccessControlException is thrown
    System.setProperty("java.version", "malicious data");

The security manager can also be installed from the command line: 
    > java -Djava.security.manager MyApp