Expected behavior

Netty works with a security manager without needing "java.lang.RuntimePermission" "accessDeclaredMembers". The ThrowableUtil class should be able to function without the addSuppressed method being available.

Actual behavior

In a environment with a security manager, Netty initialization fails due to the use of getDeclaredMethod in ThrowableUtil.

Caused by: java.security.AccessControlException: access denied ("java.lang.RuntimePermission" "accessDeclaredMembers")
    at java.base/java.security.AccessControlContext.checkPermission(AccessControlContext.java:472)
    at java.base/java.security.AccessController.checkPermission(AccessController.java:895)
    at java.base/java.lang.SecurityManager.checkPermission(SecurityManager.java:558)
    at java.base/java.lang.Class.checkMemberAccess(Class.java:2804)
    at java.base/java.lang.Class.getDeclaredMethod(Class.java:2428)
    at io.netty.util.internal.ThrowableUtil.getAddSuppressed(ThrowableUtil.java:35)
    at io.netty.util.internal.ThrowableUtil.<clinit>(ThrowableUtil.java:27)

Steps to reproduce

Run Netty with a security manager that does not grant the "accessDeclaredMembers" permission.

Reproducer

This is not minimal but see the code elastic/elasticsearch#28345. Remove the accessDeclaredMembers permission and run ./gradlew :modules:transport-netty4:check.

Netty version

4.1.18.Final in the reproducer but confirmed with 4.1.20.Final as well.

JVM version (e.g. java -version)

$ java -version
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

OS version (e.g. uname -a)

Linux build 4.13.0-25-generic #29-Ubuntu SMP Mon Jan 8 21:14:41 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux