这个问题是我之前关于自定义安全管理器生成的异常异常的问题的后续问题。在高层次上，我有兴趣构建一个运行受信任代码和不受信任代码的应用程序。我最初的想法是构建一个SecurityManager不允许大多数操作运行的自定义。这导致了异常行为，即实例化不受信任对象的受信任反射代码在 16 次调用后失败。
我已经重写了代码，因此我没有使用自定义SecurityManager来处理这个问题，而是创建了一个新的保护域，不受信任的代码在其中运行，然后从不受信任的代码中删除权限。此处显示了此新代码：
import java.io.FilePermission;
import java.lang.reflect.*;
import java.security.*;
public class Main {
/* Track how many instances have been created so that we can see when the exception
* is thrown.
*/
private static int numInstances = 0;
public Main() {
System.out.println("Number created: " + ++numInstances);
}
/* Utility function that returns a Constructor object for main. */
private static Constructor<Main> getCtor() {
try {
return Main.class.getConstructor();
} catch (NoSuchMethodException e) {
e.printStackTrace();
System.exit(-1);
return null; // Unreachable, needed to appease compiler.
}
}
/* Utility function that creates an AccessControlContext that only has file
* read permissions.
*/
private static AccessControlContext getContext() {
CodeSource c = new CodeSource(null, (java.security.cert.Certificate[])null);
Permissions permissions = new Permissions();
/* Grant specific permission to read files. This is necessary, since otherwise the
* class loader can't read classes from disk.
*/
permissions.add(new FilePermission("*", "read"));
/* Construct an AccessControlContext from these permissions. */
return new AccessControlContext(new ProtectionDomain[] {new ProtectionDomain(c, permissions)});
}
public static void main(String[] args) {
/* Get a very restrictive AccessControlContext that does not allow for anything to run. */
AccessControlContext noPermissions = getContext();
/* Install a standard security manager to enable security. */
System.setSecurityManager(new SecurityManager());
/* Sit in an infinite loop using reflection to create Main objects. This code is
* run in a context where its only permissions are file reading.
*/
AccessController.doPrivileged(new PrivilegedAction<Void>() {
@Override
public Void run() {
/* Continuously create new Main objects. */
Constructor<Main> ctor = getCtor();
try {
while (true) {
ctor.newInstance();
}
} catch (Exception e) {
e.printStackTrace();
return null;
}
}
}, noPermissions);
}
}
Main这段代码现在可以正常工作了——它可以毫无问题地构造各种对象。
我感到困惑的是以下内容。为了AccessController有任何牙齿，我们需要打开安全管理器。我通过调用来做到这一点
/* Install a standard security manager to enable security. */
System.setSecurityManager(new SecurityManager());
现在，假设我将其从默认 SecurityManager更改为自定义SecurityManager：
/* Install a standard security manager to enable security. */
System.setSecurityManager(new SecurityManager() {
@Override
public void checkPermission(Permission p) {
/* Log the permission. */
System.out.println("Checking " + p);
super.checkPermission(p);
}
});
这SecurityManager与之前相同，除了它记录检查权限时发生的情况，然后将请求转发到 default SecurityManager。
如果我进行此更改并运行程序，我现在会得到与以前相同的行为：
Checking ("java.io.FilePermission" "/home/keith/Documents/secret-eclipse-workspace/Security Manager Test/bin/Main$2.class" "read")
Checking ("java.io.FilePermission" "/home/keith/Documents/secret-eclipse-workspace/Security Manager Test/bin/Main$2.class" "read")
Checking ("java.io.FilePermission" "/home/keith/Documents/secret-eclipse-workspace/Security Manager Test/bin/Main$2.class" "read")
Number created: 1
Number created: 2
Number created: 3
Number created: 4
Number created: 5
Number created: 6
Number created: 7
Number created: 8
Number created: 9
Number created: 10
Number created: 11
Number created: 12
Number created: 13
Number created: 14
Number created: 15
Checking ("java.lang.RuntimePermission" "createClassLoader")
java.security.AccessControlException: access denied ("java.lang.RuntimePermission" "createClassLoader")
at java.security.AccessControlContext.checkPermission(AccessControlContext.java:372)
at java.security.AccessController.checkPermission(AccessController.java:559)
at java.lang.SecurityManager.checkPermission(SecurityManager.java:549)
at Main$1.checkPermission(Main.java:51)
at java.lang.SecurityManager.checkCreateClassLoader(SecurityManager.java:611)
at java.lang.ClassLoader.checkCreateClassLoader(ClassLoader.java:274)
at java.lang.ClassLoader.<init>(ClassLoader.java:316)
at sun.reflect.DelegatingClassLoader.<init>(ClassDefiner.java:72)
at sun.reflect.ClassDefiner$1.run(ClassDefiner.java:60)
at sun.reflect.ClassDefiner$1.run(ClassDefiner.java:58)
at java.security.AccessController.doPrivileged(Native Method)
at sun.reflect.ClassDefiner.defineClass(ClassDefiner.java:57)
at sun.reflect.MethodAccessorGenerator$1.run(MethodAccessorGenerator.java:399)
at sun.reflect.MethodAccessorGenerator$1.run(MethodAccessorGenerator.java:396)
at java.security.AccessController.doPrivileged(Native Method)
at sun.reflect.MethodAccessorGenerator.generate(MethodAccessorGenerator.java:395)
at sun.reflect.MethodAccessorGenerator.generateConstructor(MethodAccessorGenerator.java:94)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:48)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
at Main$2.run(Main.java:65)
at Main$2.run(Main.java:1)
at java.security.AccessController.doPrivileged(Native Method)
at Main.main(Main.java:58)
为什么我在使用此自定义之前和之后会出现不同的行为SecurityManager？我不明白为什么程序会在这些情况下产生不同的结果，因为在这两种情况下，默认值SecurityManager都是实际进行所有安全检查的那个。
谢谢！