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POConstructor.newInstance vs Class.newInstance with a SecurityManager
 

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  1. POConstructor.newInstance vs Class.newInstance with a SecurityManager
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    <p>In Java, when a SecurityManager exists that rejects access check suppression, Constructor's newInstance method works while Class's newInstance throws a SecurityException. Here's an example:</p> <pre><code>import java.lang.reflect.ReflectPermission; import java.security.Permission; public class Test { public static void main(String[] args) throws Exception { System.setSecurityManager(new SecurityManager() { @Override public void checkPermission(Permission perm) { if (perm instanceof ReflectPermission &amp;&amp; "suppressAccessChecks".equals(perm.getName())) { throw new SecurityException(); } } }); String.class.getConstructor().newInstance(); // works String.class.newInstance(); // throws SecurityException } } </code></pre> <p>Running this produces:</p> <pre><code>Exception in thread "main" java.lang.SecurityException at Test$1.checkPermission(Test.java:10) at java.lang.reflect.AccessibleObject.setAccessible(AccessibleObject.java:125) at java.lang.Class$1.run(Class.java:351) at java.security.AccessController.doPrivileged(Native Method) at java.lang.Class.newInstance0(Class.java:348) at java.lang.Class.newInstance(Class.java:325) at Test.main(Test.java:16) </code></pre> <p>The JavaDoc for <a href="http://download.oracle.com/javase/6/docs/api/java/lang/Class.html#newInstance%28%29" rel="nofollow">Class.newInstance</a> says that it calls checkMemberAccess and checkPackageAccess on the SecurityManager, but I don't know why it would call <a href="http://download.oracle.com/javase/6/docs/api/java/lang/reflect/AccessibleObject.html#setAccessible%28boolean%29" rel="nofollow">setAccessible</a>. Is there a rationale for this difference in behavior?</p> <p>I'm using:</p> <pre><code>java version "1.6.0_20" OpenJDK Runtime Environment (IcedTea6 1.9.5) (ArchLinux-6.b20_1.9.5-1-x86_64) OpenJDK 64-Bit Server VM (build 17.0-b16, mixed mode) </code></pre>
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    1. USJesse Merriman
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    1. COHave you tried reading the code/comments? This may show why it does what it does.
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      1. USPeter Lawrey
    2. COI just looked at OpenJDK's Class.java, and found this comment shortly before the setAccessible call: "Disable accessibility checks on the constructor since we have to do the security check here anyway (the stack depth is wrong for the Constructor's security check to work)". I'm still not sure why there should be a difference from the outside.
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      1. USJesse Merriman
    3. COThe comment explains it well, however to me it looks like a bug: This exception could be simply ignored, when setAccessible fails, then either it's already accessible and newInstance will work, or it isn't and newInstance will throw later. Both is correct, so ignoring the exception would be OK.
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