Acquire Java version simply

Motivation:

The Java version is used for platform dependent logic. Yet, the logic
for acquiring the Java version requires special permissions (the runtime
permission "getClassLoader") that some downstream projects will never
grant. As such, these projects are doomed to have Netty act is their
Java major version is six.  While there are ways to maintain the same
logic without requiring these special permissions, the logic is
needlessly complicated because it relies on loading classes that exist
in version n but not version n - 1. This complexity can be removed. As a
bonanza, the dangerous permission is no longer required.

Modifications:

Rather than attempting to load classes that exist in version n but not
in version n - 1, we can just parse the Java specification version. This
only requires a begign property (property permission
"java.specification.version") and is simple.

Result:

Acquisition of the Java version is safe and simple.

common/src/main/java/io/netty/util/internal/PlatformDependent.java

@@ -38,12 +38,13 @@ import java.net.InetSocketAddress;
 import java.net.ServerSocket;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
+import java.security.AccessController;
+import java.security.PrivilegedAction;
 import java.util.Deque;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Queue;
-import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentLinkedDeque;
 import java.util.concurrent.ConcurrentMap;
@@ -1018,51 +1019,48 @@ public final class PlatformDependent {
         return false;
     }
 
-    @SuppressWarnings("LoopStatementThatDoesntLoop")
     private static int javaVersion0() {
-        int javaVersion;
+        final int majorVersion;
 
-        // Not really a loop
-        for (;;) {
-            // Android
-            if (isAndroid()) {
-                javaVersion = 6;
-                break;
-            }
-
-            try {
-                Method getVersion = java.lang.Runtime.class.getMethod("version");
-                Object version = getVersion.invoke(null);
-                javaVersion = (Integer) version.getClass().getMethod("major").invoke(version);
-                break;
-            } catch (Throwable ignored) {
-                // Ignore
-            }
-
-            try {
-                Class.forName("java.time.Clock", false, getClassLoader(Object.class));
-                javaVersion = 8;
-                break;
-            } catch (Throwable ignored) {
-                // Ignore
-            }
-
-            try {
-                Class.forName("java.util.concurrent.LinkedTransferQueue", false, getClassLoader(BlockingQueue.class));
-                javaVersion = 7;
-                break;
-            } catch (Throwable ignored) {
-                // Ignore
-            }
-
-            javaVersion = 6;
-            break;
+        if (isAndroid()) {
+            majorVersion = 6;
+        } else {
+            majorVersion = majorVersionFromJavaSpecificationVersion();
         }
 
-        if (logger.isDebugEnabled()) {
-            logger.debug("Java version: {}", javaVersion);
+        logger.debug("Java version: {}", majorVersion);
+
+        return majorVersion;
+    }
+
+    static int majorVersionFromJavaSpecificationVersion() {
+        try {
+            final String javaSpecVersion = AccessController.doPrivileged(new PrivilegedAction<String>() {
+                @Override
+                public String run() {
+                    return System.getProperty("java.specification.version");
+                }
+            });
+            return majorVersion(javaSpecVersion);
+        } catch (SecurityException e) {
+            logger.debug("security exception while reading java.specification.version", e);
+            return 6;
+        }
+    }
+
+    static int majorVersion(final String javaSpecVersion) {
+        final String[] components = javaSpecVersion.split("\\.");
+        final int[] version = new int[components.length];
+        for (int i = 0; i < components.length; i++) {
+            version[i] = Integer.parseInt(components[i]);
+        }
+
+        if (version[0] == 1) {
+            assert version[1] >= 6;
+            return version[1];
+        } else {
+            return version[0];
         }
-        return javaVersion;
     }
 
     private static boolean hasUnsafe0() {

common/src/test/java/io/netty/util/internal/PlatformDependentTest.java

@@ -17,6 +17,7 @@ package io.netty.util.internal;
 
 import org.junit.Test;
 
+import java.security.Permission;
 import java.util.Random;
 
 import static io.netty.util.internal.PlatformDependent.hashCodeAscii;
@@ -129,4 +130,40 @@ public class PlatformDependentTest {
                     hashCodeAscii(string));
         }
     }
+
+    @Test
+    public void testMajorVersionFromJavaSpecificationVersion() {
+        final SecurityManager current = System.getSecurityManager();
+
+        try {
+            System.setSecurityManager(new SecurityManager() {
+                @Override
+                public void checkPropertyAccess(String key) {
+                    if (key.equals("java.specification.version")) {
+                        // deny
+                        throw new SecurityException(key);
+                    }
+                }
+
+                // so we can restore the security manager
+                @Override
+                public void checkPermission(Permission perm) {
+                }
+            });
+
+            assertEquals(6, PlatformDependent.majorVersionFromJavaSpecificationVersion());
+        } finally {
+            System.setSecurityManager(current);
+        }
+    }
+
+    @Test
+    public void testMajorVersion() {
+        assertEquals(6, PlatformDependent.majorVersion("1.6"));
+        assertEquals(7, PlatformDependent.majorVersion("1.7"));
+        assertEquals(8, PlatformDependent.majorVersion("1.8"));
+        assertEquals(8, PlatformDependent.majorVersion("8"));
+        assertEquals(9, PlatformDependent.majorVersion("1.9")); // early version of JDK 9 before Project Verona
+        assertEquals(9, PlatformDependent.majorVersion("9"));
+    }
 }