# Security Managers and the J2SE Development Kit

Last Modified: 1 May 2001

  Introduction
  Security Model Evolution
  Security Manager Evolution
    Security Manager Methods
    Security Managers in JDK 1.1
    Security Managers in the J2SE Development Kit
      Installing `java.lang.SecurityManager` as the Default Security Manager
      Changes to Methods For Class Loaders And Class Loader Depth
  How to Port 1.1-Style Security Managers
  SecurityManager Method Changes and Override Advice

# Introduction

> This document describes changes made to the security manager in the JDK that allow it to be used as-is as the default security manager in applications.

# Security Model Evolution

> In JDK 1.1, local applications and correctly digitally signed applets were generally trusted to have full access to vital system resources, such as the file system, while unsigned applets were not trusted and could access only limited resources. A security manager was responsible for determining which resource accesses were allowed.

> The J2SE Development Kit security architecture is policy-based, and allows for fine-grained access control. When code is loaded, it is assigned "permissions" based on the security policy currently in effect. Each permission specifies a permitted access to a particular resource, such as "read" and "write" access to a specified file or directory, or "connect" access to a given host and port. The policy, specifying which permissions are available for code from various signers/locations, can be initialized from an external configurable policy file. Unless a permission is explicitly granted to code, it cannot access the resource that is guarded by that permission. These new concepts of permission and policy enable the JDK to offer fine-grain, highly configurable, flexible, and extensible access control. Such access control can now not only be specified for applets, but also for all Java code, including applications, beans, and servlets.

> For more information on the Java security architecture, please see the security documentaton.

# Security Manager Evolution

> ## Security Manager Methods

> > The `SecurityManager` class contains many methods with names that begin with the word `check`. Examples are `checkRead` and `checkConnect`. Various methods in the Java libraries call a `check` method before performing each potentially security-sensitive operation. The security manager is thereby given an opportunity to prevent completion of the operation by throwing an exception. A security manager routine simply returns if the operation is permitted, but throws a SecurityException if the operation is not permitted. The only exception to this convention is `checkTopLevelWindow`, which returns a boolean value.

> > The other main type of methods contained in the `SecurityManager` class are those related to class loader existence and depth:

> > - currentClassLoader
> > - currentLoadedClass
> > - inClassLoader
> > - classLoaderDepth

> ## Security Managers in JDK 1.1

> > In JDK 1.1, the class `java.lang.SecurityManager` was an abstract class. The default implementations of the security manager `check` methods threw exceptions. The class loader and depth-related classes were appropriately implemented, often in native code.

> > Any application (such as a browser) that wanted to install a security manager had to write their own, providing appropriate concrete implementations of the methods that threw exceptions by default -- primarily the `check` methods.

> > Security managers based on the JDK 1.1 applet security manager model typically based access control decisions on two things:

> > 1. Whether or not a class with a class loader (i.e., an applet in JDK 1.1) was on the stack.

> > 2. The class loader depth -- how far down the stack was the most recent occurrence of a method from a class defined using a class loader.

> > These types of decisions were made by calling the `SecurityManager` methods related to class loader existence and depth. For example, a typical 1.1-style security manager might have a `checkExit` method like the following:

> >          public void checkExit(int status) {
> >            if (inClassLoader()) {
> >             throw new SecurityException(..);
> >            }
> >          }

> > Such a `checkExit` method would not allow `Runtime.exit` to be called when any class defined with a class loader (an applet) was on the stack. This is an example of the first case, checking whether or not a class with a class loader is on the stack. An example of the second case (class loader depth) would be something like:

> >           public void checkCreateClassLoader() {
> >              if (classLoaderDepth() == 2) {
> >                 throw new SecurityException();
> >              }
> >           }

> > This method is saying that the class loader depth can't be 2. That is, the method that called the method that called `checkCreateClassLoader` must not be in a class defined with a class loader. For example, the constructor for `java.lang.ClassLoader` calls `checkCreateClassLoader`, which means the method that calls the constructor for `java.lang.ClassLoader` must not have a class loader. This means applets can't directly create class loaders.

> > Note that there is a big difference between the two methods, even though both attempt to prevent applets from performing actions. In the first case, `checkExit` will throw an exception if an applet is anywhere on the stack. That means even built-in JDK code can't exit the VM if it was called from an applet. In the second case, JDK code is allowed to create a class loader, even if it was called by an applet. That is because the depth of a class with a class loader is used, and not the fact that there is one.

> ## Security Managers in the J2SE Development Kit

> > In the JDK, the class `java.lang.SecurityManager` had a number of changes made to it in order to allow it to be used as the default security manager for applications. In particular:

> > - It is no longer an `abstract` class, and can thus be installed as-is.

> > - Most `check` methods call a new `checkPermission` method, which by default calls the method of the same name (`checkPermission`) in the new `AccessController` class. Those methods that don't call `checkPermission` have reasonable defaults.

> > - Methods used in JDK 1.1 to determine if a class loader is on the stack and/or to calculate class loader depth have been modified in the JDK to ignore system class loaders and security contexts that have been granted `java.security.AllPermission`.

> > ## Installing `java.lang.SecurityManager` as the Default Security Manager

> > > Since `java.lang.SecurityManager` is no longer abstract, you can install and use it as the default security manager. You can do this by setting a system property when launching the VM:

> > >         java -Djava.security.manager YourApp

> > > Alternatively, your application can install it directly via the following line of code:

> > >         System.setSecurityManager(new SecurityManager());

> > > You can customize the behavior of the default security manager by modifying policy files. See the security guide on policy files for more information.

> > ## Changes to Methods For Class Loaders And Class Loader Depth

> > > In the JDK, the `SecurityManager` methods related to class loaders and class loader depth are not called by any of the `check` methods, and they are deprecated. They should not be used by any new security managers, and it is recommended that their use be eliminated from existing security managers as well. However, they are left in for backward compatibility and they have been modified in an attempt to enable old 1.1-style security managers to still work under the JDK, without modification.

> > > These methods are:

> > > - currentClassLoader
> > > - currentLoadedClass
> > > - inClassLoader
> > > - classLoaderDepth

> > > ### Modification of Class Loader/Depth-related Methods

> > > > The class loader/depth related methods have all been modified in three ways:

> > > > 1. These methods skip system class loaders. A system class loader is defined as being a class loader that is equal to the system class loader (as returned by `ClassLoader.getSystemClassLoader`) or one of its ancestors.

> > > >    Since classes loaded by the system class loader include application classes (loaded off of `CLASSPATH`), extension classes, and the built-in JDK classes, this modification enables these methods to ignore such code.

> > > >    Note that if you run an application that installs a custom security manager, and that security manager is loaded off of `CLASSPATH` in the JDK, it will have a system class loader associated with it. (Application classes did not have a class loader in JDK 1.1.) If you were to call a method like `classLoaderDepth` from within the custom security manager, and that method were not modified to ignore classes loaded by a system class loader, it would always return 0, which would not be very useful. Similarly, if class loader methods weren't changed to skip system classes and a custom security manager was loaded off of `CLASSPATH`, then this might also open up security holes in cases where the security manager is making decisions based on, for example, disallowing an operation if "classLoaderDepth() == 2". (It should really be "classLoaderDepth() <= 2".)

> > > > 2. These methods stop checking after they reach a method on the stack that has been marked as "privileged." (See java.security.AccessController.doPrivileged() and API for Privileged Blocks.)

> > > > 3. These methods treat security contexts that have been granted `AllPermission` as if there is no class loader on the stack.

> > > > As an example of the use of the first two modifications, note that there are now places in the JDK that open files, etc., after a security manager has been installed. Some 1.1-style security managers have a `checkRead` method that looks like the following:

> > > >            public void checkRead(String file) {
> > > >              if (inClassLoader()) {
> > > >               throw new SecurityException(..);
> > > >              }
> > > >            }

> > > > Without JDK code modifications, such a check run in the JDK would cause a security exception to be thrown when the JDK itself tries to read a file and there is a class with a non-system class loader on the stack. With the new security model, all such JDK code that tries to perform an operation that its caller might not be allowed to do has a `doPrivileged` block around it. Since `inClassLoader` just examines the stack up to and including the frame containing the "privileged" code, and the code at the top of the stack is JDK code, which is loaded by the system class loader or one of its ancestors, the `inClassLoader` method will return `false`, allowing the read to occur.

> > > ### Maintenance of Class Loader Depths

> > > > As noted previously, security managers based on the 1.1 applet security manager based some of their access control decisions on class loader depth. As an example, the `checkCreateClassLoader` method previously presented is repeated here:

> > > >            public void checkCreateClassLoader() {
> > > >               if (classLoaderDepth() == 2) {
> > > >                  throw new SecurityException();
> > > >               }
> > > >            }

> > > > In the JDK we have attempted to maintain the stack depth as used in 1.1-style security managers. For example, the constructor for java.security.SecureClassLoader has an explicit call to `SecurityManager.checkCreateClassLoader` even though the constructor for its super class (ClassLoader) also does. If the check was not placed in the constructor for `SecureClassLoader`, then a 1.1-style security manager would allow untrusted code to extend `SecureClassLoader` and construct class loaders, as the class loader depth would always be greater than 2.

# How to Port 1.1-Style Security Managers

> First and foremost, we highly recommend analyzing all your custom security manager methods before running your security manager under the JDK. Failure to do so could result in a security hole or prevent the proper operation of the JDK. This is due to the fragile nature of 1.1-style security managers.

> Where possible, you should just use the default implementation of the 1.2 `SecurityManager`. This helps give users and administrators consistent behavior. If this is not possible, then you should at least try to call `super.checkXXX` in your `checkXXX` method before throwing a security exception. Doing so will allow the access controller algorithm to be used, and will allow the JDK itself to function correctly.

> In the JDK, any existing code that used to call any of the `SecurityManager check` methods continues to do so. For new code that requires a security check, calls are made to `SecurityManager.checkPermission` instead of adding a new `SecurityManager check` method. For example, the new `java.lang.System.setProperty` method calls `checkPermission` with a `java.util.PropertyPermission` permission.

> When extending the SecurityManager class and overriding existing methods, some care should be taken. For example, if you override the `checkRead(String file)` method so it always throws a security exception, then the JDK itself may fail to operate properly. That is, if some JDK code needs to open a file (to read a properties file, load a JAR file, etc.) then throwing a security exception for every read attempt would cause such opens to always fail.

> In general, you should only override the default methods if you intend to loosen security, not to make it stronger. If you want to tighten security, you should modify the default policy files and/or install a custom `java.security.Policy` object. See the security guide on policy files for more information.

> In general, when overriding security manager methods you should place a call to the `super.checkXXX` method at the point where your overridden `checkXXX` method would throw an exception. For example:

>           public class MySecurityManager extends SecurityManager {

>             public void checkRead(String file) {
>               if (someCustomSecurityCheckFails()) {
>                  super.checkRead(file);
>               }
>             }
>           }

> If your custom security check fails, then `super.checkRead` gets called. The default implementation of `checkRead` invokes `checkPermission`, which by default consults the `AccessController`. By invoking the `AccessController`, system code that has done an `AccessController.doPrivileged` before trying to read a file will succeed in reading that file. All other code will be subjected to the current policy in effect, and an access control exception will be thrown if access to that file has not been granted.

> Note, there are some `checkXXX` methods in which you should not call `super.checkXXX` methods when overriding them. That is because the default implementation of these methods may not be as strict as the policy you are implementing in the overridden method. For example, the default `checkAccess(ThreadGroup g)` method only protects the system thread group. If you intend to protect threads in distinct thread groups from each other (for example applet thread groups), then you would not want to call `super.checkAccess` at the point you would normally throw a security exception, as that would defeat the purpose of your customized check. Instead, you could place a call to `super.checkAccess` as the first statement in your overridden method.

> For example:

>           public class AppletSecurityManager extends SecurityManager {

>             public void checkAccess(ThreadGroup g) {
>               // a call to super will throw an exception if someone
>               // is trying to modify the system thread group
>               super.checkAccess(g);
>               ...
>               // now perform checks based on which applet thread group
>               // the current caller is in to see if they can modify thread group g.
>               ...
>           }

> We describe how to override each method in the following section.

# SecurityManager Method Changes and Override Advice

> This section lists changes made to `java.lang.SecurityManager` methods in the JDK and provides suggestions regarding any overrides you may wish to make. Please see the Java documentation for the `SecurityManager` class for more information on these methods.

> ### protected boolean inCheck

> > This field has been deprecated, and any uses of this field within the JDK itself have been removed. Instead of using inCheck, you should use `checkPermission` along with `doPrivileged`.

> ### public boolean getInCheck();

> > This method has also been deprecated.

> ### public SecurityManager();

> > The constructor has been modified to allow multiple SecurityManagers to be created, assuming the caller has the `RuntimePermission("createSecurityManager")` permission.

> ### protected native Class[] getClassContext();

> > No changes. This call can be used to emulate the 1.1 behavior of the methods that have been changed in the JDK ( `currentClassLoader`, `currentLoadedClass`, `classLoaderDepth`, `inClassLoader`).

> ### protected ClassLoader currentClassLoader();

> > The typical use of this method in JDK 1.1-style security managers was to see if there was a class loader on the stack, and if not, treat the code as "trusted" and allow it to do anything. This method has been modified in the JDK to allow trusted JDK code (actually any code granted `java.security.AllPermission`) that calls `doPrivileged` to be treated as trusted by 1.1-style security managers. It has also been modified to skip system class loaders. A system class loader is defined as being a class loader that is equal to the system class loader (as returned by ClassLoader.getSystemClassLoader) or one of its ancestors.

> > This method will return `null` in the following three cases:

> > 1. All methods on the execution stack are from classes defined using the system class loader or one of its ancestors.
> > 2. All methods on the execution stack up to the first "privileged" caller (see java.security.AccessController.doPrivileged) are from classes defined using the system class loader or one of its ancestors.
> > 3. A call to checkPermission with `java.security.AllPermission` does not result in a SecurityException.

> > This method has been deprecated. Use `checkPermission` instead.

> ### protected Class currentLoadedClass();

> > This method has been modified in the same fashion as `currentClassLoader`, and will return `null` if the current security context has been granted `AllPermission` or all the methods on the stack (up to the first privileged caller, if any) are from classes defined using the system class loader or one of its ancestors.

> > This method has been deprecated. Use `checkPermission` instead.

> ### protected int classDepth(String name);

> > No changes in behavior. This method has been deprecated. Use `checkPermission` instead.

> ### protected int classLoaderDepth();

> > This method has been modified in the same fashion as `currentClassLoader`, and will return `-1` if the current security context has been granted `AllPermission` or all the methods on the stack (up to the first privileged caller, if any) are from classes defined using the system class loader or one of its ancestors.

> > This method has been deprecated. Use `checkPermission` instead.

> ### protected boolean inClass(String name);

> > No changes in behavior. This method has been deprecated. Use `checkPermission` instead.

> ### protected boolean inClassLoader();

> > This method returns true if `currentClassLoader` returns a non-null class loader, so it follows the same semantics that `currentClassLoader` does.

> > This method has been deprecated. Use `checkPermission` instead.

> ### public Object getSecurityContext();

> > This method returns a `java.security.AccessControlContext` object that is created with a call to `java.security.AccessController.getContext`. In JDK1.1 it returned `null` by default.

> ### public void checkPermission(Permission perm);

> > This method is new in the JDK. It calls `java.security.AccessController.checkPermission` with the given permission. Internally, the JDK always calls `SecurityManager.checkPermission` instead of calling the `AccessController` directly. This allows people to override this method to provide additional functionality such as auditing and/or GUI dialogs.

> ### public void checkPermission(Permission perm, Object context);

> > This method is new in the JDK. If `context` is an instance of `AccessControlContext` then the `AccessControlContext.checkPermission` method will be invoked on the given context with the specified permission.

> > If `context` is not an instance of `AccessControlContext` then a `SecurityException` is thrown.

> ### public void checkCreateClassLoader();

> > This method has been modified to call `checkPermission` with the `RuntimePermission("createClassLoader")` permission.

> > If this method is overridden, then a call to `super.checkCreateClassLoader` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkCreateClassLoader() {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkCreateClassLoader();
> >           }
> >         }
> >       }

> ### public void checkAccess(Thread t);

> > If the thread argument is a system thread (belongs to the thread group with a `null` parent) then this method calls `checkPermission` with the `RuntimePermission("modifyThread")` permission.

> > Applications that want a stricter policy should override this method.

> > If this method is overridden, then `super.checkAccess` should be called by the first statement in the overridden method, or the equivalent security check should be placed in the overridden method.

> > If this method is overridden, the method that overrides it should additionally check to see if the calling thread has the `RuntimePermission("modifyThread")` permission, and if so, return silently. This is to ensure that code granted that permission (such as the JDK itself) is allowed to manipulate any thread.

> > For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkAccess(Thread t) {
> >           // a call to super will throw an exception if someone
> >           // is trying to modify a system thread
> >           super.checkAccess(t);
> >           ...
> >           if (someCustomSecurityCheckForOtherThreadsFails()) {
> >             // if the check fails, instead of throwing an exception,
> >             // call checkPermission, which will throw an exception
> >             // if need be
> >             checkPermission(new RuntimePermission("modifyThread"));
> >           }
> >           ...
> >         }
> >       }

> ### public void checkAccess(ThreadGroup g);

> > If the thread group argument is the system thread group (has a `null` parent) then this method calls `checkPermission` with the `RuntimePermission("modifyThreadGroup")` permission.

> > Applications that want a stricter policy should override this method.

> > If this method is overridden, then `super.checkAccess` should be called by the first statement in the overridden method, or the equivalent security check should be placed in the overridden method.

> > If this method is overridden, the method that overrides it should additionally check to see if the caller has the `RuntimePermission("modifyThreadGroup")` permission, and if so, return silently. This is to ensure that code granted that permission (such as the JDK itself) is allowed to manipulate any thread group.

> > For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkAccess(ThreadGroup g) {
> >           // a call to super will throw an exception if someone
> >           // is trying to modify the system thread group
> >           super.checkAccess(g);
> >           ...
> >           if (someCustomSecurityCheckForOtherThreadGroupsFails()) {
> >             // if the check fails, instead of throwing an exception,
> >             // call checkPermission, which will throw an exception
> >             // if need be
> >             checkPermission(new RuntimePermission("modifyThreadGroup"));
> >           }
> >           ...
> >         }
> >       }

> ### public void checkExit(int status);

> > This method has been modified to call `checkPermission` with the `RuntimePermission("exitVM")` permission.

> > If this method is overridden, then a call to `super.checkExit` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkExit(int status) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkExit(status);
> >           }
> >         }
> >       }

> ### public void checkExec(String cmd);

> > This method has been modified to call `checkPermission` with a `FilePermission`. If the `cmd` is an absolute path (see `java.io.File.isAbsolute`) then it is passed as-is as the target for the `FilePermission`. If `cmd` is not absolute, then the special target "<<ALL FILES>>" is used. This target is used because it is difficult to determine the actual path of the command that will be executed on an individual platform due to things such as environment variables, etc.

> > If this method is overridden, then a call to `super.checkExec` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkExec(String cmd) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkExec(cmd);
> >           }
> >         }
> >       }

> ### public void checkLink(String lib);

> > This method has been modified to call `checkPermission` with the `RuntimePermission("loadLibrary."+lib)` permission.

> > If this method is overridden, then a call to `super.checkLink` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkLink(String lib) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkLink(lib);
> >           }
> >         }
> >       }

> ### public void checkRead(FileDescriptor fd);

> > This method has been modified to call `checkPermission` with the `RuntimePermission("readFileDescriptor")` permission.

> > If this method is overridden, then a call to `super.checkRead` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkRead(FileDescriptor fd) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkRead(fd);
> >           }
> >         }
> >       }

> ### public void checkRead(String file);

> > This method has been modified to call `checkPermission` with the `FilePermission(file,"read")` permission.

> > If this method is overridden, then a call to `super.checkRead` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkRead(String file) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkRead(file);
> >           }
> >         }
> >       }

> ### public void checkRead(String file, Object context);

> > This method has been modified. If `context` is an instance of `AccessControlContext` then the `AccessControlContext.checkPermission` method will be invoked on the given context with the `FilePermission(file,"read")` permission.

> > If `context` is not an instance of `AccessControlContext` then a `SecurityException` is thrown.

> > If this method is overridden, then a call to `super.checkRead` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkRead(String file, Object context) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkRead(file, context);
> >           }
> >         }
> >       }

> ### public void checkWrite(FileDescriptor fd);

> > This method has been modified to call `checkPermission` with the `RuntimePermission("writeFileDescriptor")` permission.

> > If this method is overridden, then a call to `super.checkWrite` should be made at the point the overridden method would normally throw an exception. For Example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkWrite(FileDescriptor fd) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkWrite(fd);
> >           }
> >         }
> >       }

> ### public void checkWrite(String file);

> > This method has been modified to call `checkPermission` with the `FilePermission(file,"write")` permission.

> > If this method is overridden, then a call to `super.checkWrite` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkWrite(String file) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkWrite(file);
> >           }
> >         }
> >       }

> ### public void checkDelete(String file);

> > This method has been modified to call `checkPermission` with the `FilePermission(file,"delete")` permission.

> > If this method is overridden, then a call to `super.checkDelete` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkDelete(String file) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkDelete(file);
> >           }
> >         }
> >       }

> ### public void checkConnect(String host, int port);

> > This method has been modified to call `checkPermission` with the `SocketPermission(host+":"+port,"connect")` permission if the port is not equal to -1. If the port is equal to -1, then it calls `checkPermission` with the `SocketPermission(host,"resolve")` permission.

> > This behavior is consistent with JDK 1.1, where a port equal to -1 indicates that an IP address lookup is being performed.

> > If this method is overridden, then a call to `super.checkConnect` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkConnect(String host, int port) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkConnect(host, port);
> >           }
> >         }
> >       }

> ### public void checkConnect(String host, int port, Object context);

> > This method has been modified. If `context` is an instance of `AccessControlContext` then the `AccessControlContext.checkPermission` method will be invoked on the given context with the `SocketPermission(host+":"+port,"connect")` permission if the port is not equal to -1. If the port is equal to -1, then it calls `checkPermission` with the `SocketPermission(host,"resolve")` permission.

> > If `context` is not an instance of `AccessControlContext` then a `SecurityException` is thrown.

> > If this method is overridden, then a call to `super.checkConnect` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkConnect(String host, int port, Object context) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkConnect(host, port, context);
> >           }
> >         }
> >       }

> ### public void checkListen(int port)

> > This method has been modified. If port is not 0, it calls `checkPermission` with the `SocketPermission("localhost:"+port,"listen")`. If port is zero, it calls `checkPermission` with `SocketPermission("localhost:1024-","listen").`

> > If this method is overridden, then a call to `super.checkListen` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkListen(int port) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkListen(port);
> >           }
> >         }
> >       }

> ### public void checkAccept(String host, int port);

> > This method has been modified to call `checkPermission` with the `SocketPermission(host+":"+port,"accept")` permission.

> > If this method is overridden, then a call to `super.checkAccept` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkAccept(String host, int port) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkAccept(host, port);
> >           }
> >         }
> >       }

> ### public void checkMulticast(InetAddress maddr);

> > This method has been modified to call `checkPermission` with the `SocketPermission(maddr.getHostAddress(),"accept,connect")` permission.

> > If this method is overridden, then a call to `super.checkMulticast` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkMultiCast(InetAddress maddr) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkMultiCast(maddr);
> >           }
> >         }
> >       }

> ### public void checkMulticast(InetAddress maddr, byte ttl);

> > This method has been modified to call `checkPermission` with the `SocketPermission(maddr.getHostAddress(),"accept,connect")` permission.

> > If this method is overridden, then a call to `super.checkMulticast` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkMultiCast(InetAddress maddr, byte ttl) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkMultiCast(maddr, ttl);
> >           }
> >         }
> >       }

> ### public void checkPropertiesAccess();

> > This method has been modified to call `checkPermission` with the `PropertyPermission("*", "read,write")` permission.

> > If this method is overridden, then a call to `super.checkPropertiesAccess` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkPropertiesAccess() {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkPropertiesAccess();
> >           }
> >         }
> >       }

> ### public void checkPropertyAccess(String key);

> > This method has been modified to call `checkPermission` with the `PropertyPermission(key, "read")` permission.

> > If this method is overridden, then a call to `super.checkPropertyAccess` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkPropertyAccess(String key) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkPropertiesAccess(key);
> >           }
> >         }
> >       }

> ### public boolean checkTopLevelWindow(Object window);

> > This method has been modified to call `checkPermission` with the `AWTPermission("showWindowWithoutWarningBanner")` permission, and returns true if an SecurityException is not thrown, otherwise it returns false.

> > If this method is overridden, then a call to `super.checkTopLevelWindow` should be made at the point the overridden method would normally return false, and the value of `super.checkTopLevelWindow` should be returned. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkTopLevelWindow(Object window) {
> >           if (someCustomSecurityCheckFails()) {
> >             return super.checkTopLevelWindow(window);
> >           } else {
> >             return true;
> >           }
> >         }
> >       }

> ### public void checkPrintJobAccess();

> > This method has been modified to call `checkPermission` with the `RuntimePermission("queuePrintJob")` permission.

> > If this method is overridden, then a call to `super.checkPrintJobAccess` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkPrintJobAccess() {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkPrintJobAccess();
> >           }
> >         }
> >       }

> ### public void checkSystemClipboardAccess();

> > This method has been modified to call `checkPermission` with the `AWTPermission("accessClipboard")` permission.

> > If this method is overridden, then a call to `super.checkSystemClipboardAccess` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkSystemClipboardAccess() {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkSystemClipboardAccess();
> >           }
> >         }
> >       }

> ### public void checkAwtEventQueueAccess();

> > This method has been modified to call `checkPermission` with the `AWTPermission("accessEventQueue")` permission.

> > If this method is overridden, then a call to `super.checkAwtEventQueueAccess` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkAwtEventQueueAccess() {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkAwtEventQueueAccess();
> >           }
> >         }
> >       }

> ### public void checkPackageAccess(String pkg);

> > This method has been modified. It first gets a list of restricted packages by obtaining a comma-separated list from a call to `java.security.Security.getProperty("package.access")`, and checks to see if `pkg` starts with or equals any of the restricted packages. If it does, then `checkPermission` gets called with the `RuntimePermission("accessClassInPackage."+pkg)` permission.

> > If this method is overridden, then `super.checkPackageAccess` should be called as the first line in the overridden method. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkPackageAccess(String pkg) {
> >           super.checkPackageAccess(pkg);
> >           ...
> >           someCustomSecurityCheck();
> >           ...
> >         }
> >       }

> ### public void checkPackageDefinition(String pkg);

> > This method has been modified. It first gets a list of restricted packages by obtaining a comma-separated list from a call to `java.security.Security.getProperty("package.definition")`, and checks to see if `pkg` starts with or equals any of the restricted packages. If it does, then `checkPermission` gets called with the `RuntimePermission("defineClassInPackage."+pkg)` permission.

> > If this method is overridden, then `super.checkPackageDefinition` should be called as the first line in the overridden method. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkPackageDefinition(String pkg) {
> >           super.checkPackageDefinition(pkg);
> >           ...
> >           someCustomSecurityCheck();
> >           ...
> >         }
> >       }

> ### public void checkSetFactory();

> > This method has been modified to call `checkPermission` with the `RuntimePermission("setFactory")` permission.

> > If this method is overridden, then a call to `super.checkSetFactory` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkSetFactory() {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkSetFactory();
> >           }
> >         }
> >       }

> ### public void checkMemberAccess(Class clazz, int which);

> > This method has been modified. The default policy is to allow access to PUBLIC members, as well as access to classes that have the same class loader as the caller. In all other cases call `checkPermission` with the `RuntimePermission("accessDeclaredMembers")` permission.

> > If this method is overridden, then a call to `super.checkMemberAccess` cannot be made, as the default implementation of `checkMemberAccess` relies on the code being checked being at a stack depth of 4. For example:

> >          someCaller[3]
> >          java.lang.Class.someReflectionAPI [2]
> >          java.lang.Class.checkMemberAccess [1]
> >          SecurityManager.checkMemberAccess [0]

> > In order to emulate this behavior, you would need to call `getClassContext`, and examine the class loader of the class at index 3, just as the default `checkMemberAccess` method does:

> >       if (which != Member.PUBLIC) {
> >         Class stack[] = getClassContext();
> >         /*
> >          * stack depth of 4 should be the caller of one of the
> >          * methods in java.lang.Class that invoke checkMember
> >          * access. The stack should look like:
> >          *
> >          * someCaller [3]
> >          * java.lang.Class.someReflectionAPI  [2]
> >          * java.lang.Class.checkMemberAccess [1]
> >          * MySecurityManager.checkMemberAccess [0]
> >          *
> >          */
> >         if ((stack.length<4) ||
> >           (stack[3].getClassLoader() != clazz.getClassLoader())) {
> >           if (checkMemberAccessPermission == null)
> >             checkMemberAccessPermission =
> >               new RuntimePermission("accessDeclaredMembers");
> >           checkPermission(checkMemberAccessPermission);
> >         }
> >       }

> > This is the only security manager method in the JDK that is still based on a caller's depth. This is to allow a caller to reflect on classes from the same class loader it came from.

> ### public void checkSecurityAccess(String target);

> > This method has been modified to create a `SecurityPermission` object for the given permission target name and calls `checkPermission` with it.

> > If this method is overridden, then a call to `super.checkSecurityAccess` should be made at the point the overridden method would normally throw an exception. For example:

> >       public class MySecurityManager extends SecurityManager {

> >         public void checkSecurityAccess(String target) {
> >           if (someCustomSecurityCheckFails()) {
> >             super.checkSecurityAccess(target);
> >           }
> >         }
> >       }

> ### public ThreadGroup getThreadGroup();

> > This method has not been changed.

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