Java Security, 2nd Edition

Overview of the Security Manager

When most people think of Java security, they think of the protections afforded to a Java program -- and, more particularly, only by default to a Java applet -- by Java’s security manager. There are other important facets of Java’s security story, but the role played by the security manager is of paramount importance in defining the security policy under which a particular program will operate.

On one level, the Java security manager is simple to understand, and it’s often summarized by saying that it prevents Java applets from accessing your local disk or local network. The real story is more complicated than that, however, with the result that Java’s security manager is often misunderstood.

On a simple level, the security manager is responsible for determining most of the parameters of the Java sandbox -- that is, it is ultimately up to the security manager to determine whether many particular operations should be permitted or rejected. If a Java program attempts to open a file, the security manager decides whether or not that operation should be permitted. If a Java program wants to connect to a particular machine on the network, it must first ask permission of the security manager. If a Java program wants to alter the state of certain threads, the security manager will intervene if such an operation is considered dangerous.

The security manager is used only if it is explicitly installed. When you run a Java application, specifying the -Djava.security.manager ...