Java Security Manager - What does it check?

问题

This article about Java security says:

Code in the Java library consults the Security Manager whenever a dangerous operation is about to be attempted.

So, what does this exactly mean? Say, if I've implemented my own securitymanager and enabled it for the whole JVM. Now, does the java runtime consults my securitymanager for each and every java call(like System.out.println() etc) or it consults only for dangerous api calls like System.exit() ,file operations etc?

edit: let me clarify my question,

I'm not questioning the possiblities of the securitymanager. I'm just asking if the security checks are done for the dangerous api's alone or it is done for each and every method call. Which inturn causes a huge performance degradation in case of applications with large amounts of code.

回答1:

It will only consult the SecurityManager if the code says so. It won't do it for every single operation.

For example in Runtime.exit, you see that the SecurityManager is consulted:

public void exit(int status) {
SecurityManager security = System.getSecurityManager();
if (security != null) {
    security.checkExit(status);
}
Shutdown.exit(status);
}

Similarly, in File, you will see that most methods consult the SecurityManager. Example:

public boolean canWrite() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
    security.checkWrite(path);
}
return fs.checkAccess(this, FileSystem.ACCESS_WRITE);
}

If you are writing a method which might be "dangerous" then you should also consult the SecurityManager.

回答2:

Using security manager you could control access to :

File operations
Reflection facility
Read/Write IO
Thread/Thread group operations
Socket operations(listen, accept etc.)
Power to create your own classloader.

For each such thing there is a check*() method in SecurityManager

For an exhaustive list check the constants in SecurityConstants

回答3:

The security manager uses a policy file to see what is permitted and what's not permitted. "Dangerous" operations, as determined by this policy file, is granted or denied during the execution.

You can find more details about the default policy for Sun/Oracle JVM here:

http://download.oracle.com/javase/6/docs/technotes/guides/security/PolicyFiles.html

来源：https://stackoverflow.com/questions/5192965/java-security-manager-what-does-it-check

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