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8  * particular file as subject to the "Classpath" exception as provided
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13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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25 
26 package jdk.dynalink.linker;
27 
28 import java.security.Permission;
29 import java.util.List;
30 import java.util.ServiceLoader;
31 import java.util.function.Supplier;
32 import jdk.dynalink.DynamicLinkerFactory;
33 
34 /**
35  * A class acting as a supplier of guarding dynamic linkers that can be
36  * automatically loaded by other language runtimes. Language runtimes wishing
37  * to export their own linkers should subclass this class and implement the
38  * {@link #get()} method to return a list of exported linkers and declare the
39  * subclass in
40  * {@code /META-INF/services/jdk.dynalink.linker.GuardingDynamicLinkerExporter}
41  * resource of their distribution (typically, JAR file) so that dynamic linker
42  * factories can discover them using the {@link ServiceLoader} mechanism. Note
43  * that instantiating this class is tied to a security check for the
44  * {@code RuntimePermission("dynalink.exportLinkersAutomatically")} when a
45  * security manager is present, to ensure that only trusted runtimes can
46  * automatically export their linkers into other runtimes.
47  * @see DynamicLinkerFactory#setClassLoader(ClassLoader)
48  */
49 public abstract class GuardingDynamicLinkerExporter implements Supplier<List<GuardingDynamicLinker>> {
50     /**
51      * The name of the runtime permission for creating instances of this class.
52      * Granting this permission to a language runtime allows it to export its
53      * linkers for automatic loading into other language runtimes.
54      */
55     public static final String AUTOLOAD_PERMISSION_NAME = "dynalink.exportLinkersAutomatically";
56 
57     private static final Permission AUTOLOAD_PERMISSION = new RuntimePermission(AUTOLOAD_PERMISSION_NAME);
58 
59     /**
60      * Creates a new linker exporter. If there is a security manager installed
61      * checks for the
62      * {@code RuntimePermission("dynalink.exportLinkersAutomatically")} runtime
63      * permission. This ensures only language runtimes granted this permission
64      * will be allowed to export their linkers for automatic loading.
65      * @throws SecurityException if the necessary runtime permission is not
66      * granted.
67      */
GuardingDynamicLinkerExporter()68     protected GuardingDynamicLinkerExporter() {
69         final SecurityManager sm = System.getSecurityManager();
70         if (sm != null) {
71             sm.checkPermission(AUTOLOAD_PERMISSION);
72         }
73     }
74 }
75