diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 61fa67d04..6617a3f65 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -21,6 +21,7 @@ jobs:
         os: [ ubuntu-latest, macos-latest, windows-latest ]
         experimental: [false]
         include:
+          - { ruby: jruby-head,       os: ubuntu-latest,  experimental: true }
           - { ruby: truffleruby-head, os: ubuntu-latest,  experimental: true }
     runs-on: ${{ matrix.os }}
     continue-on-error: ${{ matrix.experimental }}
diff --git a/rakelib/string_prep_tables_generator.rb b/rakelib/string_prep_tables_generator.rb
index 44f2ce88e..e9132bbcf 100644
--- a/rakelib/string_prep_tables_generator.rb
+++ b/rakelib/string_prep_tables_generator.rb
@@ -292,6 +292,12 @@ def parse_rfc_text(rfc3454_text)
       .to_h.compact
       .transform_values {|t| t.first.size == 2 ? t.to_h : t }
     tables["titles"] = titles
+
+    # See https://github.com/ruby/json/issues/870
+    if RUBY_ENGINE == "jruby"
+      tables["titles"].transform_values! { _1.dump.undump }
+    end
+
     tables
   end
 
diff --git a/test/net/imap/test_data_lite.rb b/test/net/imap/test_data_lite.rb
index 42681a0e1..69352f7fa 100644
--- a/test/net/imap/test_data_lite.rb
+++ b/test/net/imap/test_data_lite.rb
@@ -174,30 +174,32 @@ def initialize(value:, head: nil, tail: nil)
           end
         end
 
-        # anonymous class
-        list = klass[value: 1, tail: [2, 3, 4]]
-        seen = "#<data #{klass.inspect}:...>"
-        assert_equal(
-          "#<data value=1, head=nil," \
-          " tail=#<data value=2, head=#{seen}," \
-          " tail=#<data value=3, head=#{seen}," \
-          " tail=#<data value=4, head=#{seen}," \
-          " tail=nil>>>>",
-          list.inspect
-        )
-
-        # named class
-        Object.const_set(:DoubleLinkList, klass)
-        list = DoubleLinkList[value: 1, tail: [2, 3, 4]]
-        seen = "#<data DoubleLinkList:...>"
-        assert_equal(
-          "#<data DoubleLinkList value=1, head=nil," \
-          " tail=#<data DoubleLinkList value=2, head=#{seen}," \
-          " tail=#<data DoubleLinkList value=3, head=#{seen}," \
-          " tail=#<data DoubleLinkList value=4, head=#{seen}," \
-          " tail=nil>>>>",
-          list.inspect
-        )
+        pend_if_jruby do
+          # anonymous class
+          list = klass[value: 1, tail: [2, 3, 4]]
+          seen = "#<data #{klass.inspect}:...>"
+          assert_equal(
+            "#<data value=1, head=nil," \
+            " tail=#<data value=2, head=#{seen}," \
+            " tail=#<data value=3, head=#{seen}," \
+            " tail=#<data value=4, head=#{seen}," \
+            " tail=nil>>>>",
+            list.inspect
+          )
+
+          # named class
+          Object.const_set(:DoubleLinkList, klass)
+          list = DoubleLinkList[value: 1, tail: [2, 3, 4]]
+          seen = "#<data DoubleLinkList:...>"
+          assert_equal(
+            "#<data DoubleLinkList value=1, head=nil," \
+            " tail=#<data DoubleLinkList value=2, head=#{seen}," \
+            " tail=#<data DoubleLinkList value=3, head=#{seen}," \
+            " tail=#<data DoubleLinkList value=4, head=#{seen}," \
+            " tail=nil>>>>",
+            list.inspect
+          )
+        end
       ensure
         Object.instance_eval { remove_const(:DoubleLinkList) } rescue nil
       end
@@ -362,7 +364,9 @@ class InheritsClassMethod < AbstractWithClassMethod.define(:foo)
       end
 
       def test_subclass_class_method
-        assert_equal :ok, InheritsClassMethod.inherited_class_method
+        pend_if_jruby do
+          assert_equal :ok, InheritsClassMethod.inherited_class_method
+        end
       end
 
       class AbstractWithOverride < Data
diff --git a/test/net/imap/test_fetch_data.rb b/test/net/imap/test_fetch_data.rb
index 37c074bbf..adb0dea0d 100644
--- a/test/net/imap/test_fetch_data.rb
+++ b/test/net/imap/test_fetch_data.rb
@@ -25,14 +25,14 @@ def fetch_data_class
   end
 
   test "#seqno does not exist" do
-    data = Net::IMAP::UIDFetchData.new(22222)
+    data = pend_if_jruby { Net::IMAP::UIDFetchData.new(22222) } or next
     assert_raise NoMethodError do
       data.seqno
     end
   end
 
   test "#uid replaces #seqno" do
-    data = Net::IMAP::UIDFetchData.new(22222)
+    data = pend_if_jruby { Net::IMAP::UIDFetchData.new(22222) } or next
     assert_equal 22222, data.uid
   end
 
diff --git a/test/net/imap/test_imap.rb b/test/net/imap/test_imap.rb
index 46047a294..95375b18b 100644
--- a/test/net/imap/test_imap.rb
+++ b/test/net/imap/test_imap.rb
@@ -13,6 +13,7 @@ class IMAPTest < Net::IMAP::TestCase
 
   if defined?(OpenSSL::SSL::SSLError)
     def test_imaps_unknown_ca
+      omit_if_jruby "SSL tests don't work yet"
       assert_raise(OpenSSL::SSL::SSLError) do
         imaps_test do |port|
           begin
@@ -27,6 +28,7 @@ def test_imaps_unknown_ca
     end
 
     def test_imaps_with_ca_file
+      omit_if_jruby "SSL tests don't work yet"
       # Assert verified *after* the imaps_test and assert_nothing_raised blocks.
       # Otherwise, failures can't logout and need to wait for the timeout.
       verified, imap = :unknown, nil
@@ -54,6 +56,7 @@ def test_imaps_with_ca_file
     end
 
     def test_imaps_verify_none
+      omit_if_jruby "SSL tests don't work yet"
       # Assert verified *after* the imaps_test and assert_nothing_raised blocks.
       # Otherwise, failures can't logout and need to wait for the timeout.
       verified, imap = :unknown, nil
@@ -81,6 +84,7 @@ def test_imaps_verify_none
     end
 
     def test_imaps_post_connection_check
+      omit_if_jruby "SSL tests don't work yet"
       assert_raise(OpenSSL::SSL::SSLError) do
         imaps_test do |port|
           # server_addr is different from the hostname in the certificate,
@@ -95,6 +99,7 @@ def test_imaps_post_connection_check
 
   if defined?(OpenSSL::SSL)
     def test_starttls_unknown_ca
+      omit_if_jruby "SSL tests don't work yet"
       omit "This test is not working with Windows" if RUBY_PLATFORM =~ /mswin|mingw/
 
       imap = nil
@@ -115,6 +120,7 @@ def test_starttls_unknown_ca
     end
 
     def test_starttls
+      omit_if_jruby "SSL tests don't work yet"
       initial_verified, initial_ctx, initial_params = :unknown, :unknown, :unknown
       imap = nil
       starttls_test do |port|
@@ -139,6 +145,7 @@ def test_starttls
     end
 
     def test_starttls_stripping
+      omit_if_jruby "SSL tests don't work yet"
       imap = nil
       starttls_stripping_test do |port|
         imap = Net::IMAP.new("localhost", :port => port)
@@ -497,6 +504,7 @@ def test_connection_closed_during_idle
   end
 
   def test_connection_closed_without_greeting
+    omit_if_jruby "???"
     server = create_tcp_server
     port = server.addr[1]
     h = {
diff --git a/test/net/imap/test_imap_capabilities.rb b/test/net/imap/test_imap_capabilities.rb
index c2274798c..9d605d57c 100644
--- a/test/net/imap/test_imap_capabilities.rb
+++ b/test/net/imap/test_imap_capabilities.rb
@@ -124,6 +124,7 @@ class IMAPCapabilitiesTest < Net::IMAP::TestCase
 
   if defined?(OpenSSL::SSL::SSLError)
     test "#capabilities caches greeting capabilities (implicit TLS)" do
+      omit_if_jruby
       with_fake_server(preauth: false, implicit_tls: true) do |server, imap|
         assert imap.capabilities_cached?
         assert_equal %w[IMAP4REV1 AUTH=PLAIN], imap.capabilities
@@ -136,6 +137,7 @@ class IMAPCapabilitiesTest < Net::IMAP::TestCase
 
     test "#capabilities cache is cleared after #starttls" do
       with_fake_server(preauth: false, cleartext_auth: false) do |server, imap|
+        omit_if_jruby
         assert imap.capabilities_cached?
         assert imap.capable? :IMAP4rev1
         refute imap.auth_capable? "plain"
@@ -189,6 +191,7 @@ class IMAPCapabilitiesTest < Net::IMAP::TestCase
 
   # TODO: should we warn about this?
   test "#capabilities cache IGNORES tagged OK response to STARTTLS" do
+    omit_if_jruby
     with_fake_server(preauth: false) do |server, imap|
       server.on "STARTTLS" do |cmd|
         cmd.done_ok code: "[CAPABILITY IMAP4rev1 AUTH=PLAIN fnord]"
diff --git a/test/net/imap/test_imap_deprecated_client_options.rb b/test/net/imap/test_imap_deprecated_client_options.rb
index 9ff9ad23e..b02b49bdc 100644
--- a/test/net/imap/test_imap_deprecated_client_options.rb
+++ b/test/net/imap/test_imap_deprecated_client_options.rb
@@ -42,6 +42,7 @@ class InitializeTests < IMAPDeprecatedClientOptionsTest
     end
 
     test "Convert deprecated usessl (= true) and certs, with warning" do
+      omit_if_jruby "SSL tests don't work yet"
       run_fake_server_in_thread(implicit_tls: true) do |server|
         certs = server.config.tls[:ca_file]
         assert_deprecated_warning(/Call Net::IMAP\.new with keyword/i) do
@@ -58,6 +59,7 @@ class InitializeTests < IMAPDeprecatedClientOptionsTest
     end
 
     test "Convert deprecated usessl (= true) and verify (= false), with warning" do
+      omit_if_jruby "SSL tests don't work yet"
       run_fake_server_in_thread(implicit_tls: true) do |server|
         assert_deprecated_warning(/Call Net::IMAP\.new with keyword/i) do
           with_client("localhost", server.port, true, nil, false) do |client|
@@ -89,6 +91,7 @@ class InitializeTests < IMAPDeprecatedClientOptionsTest
 
   class StartTLSTests < IMAPDeprecatedClientOptionsTest
     test "Convert obsolete options hash to keywords" do
+      omit_if_jruby "SSL tests don't work yet"
       with_fake_server(preauth: false) do |server, imap|
         imap.starttls(ca_file: server.config.tls[:ca_file], min_version: :TLS1_2)
         assert_equal(
@@ -101,6 +104,7 @@ class StartTLSTests < IMAPDeprecatedClientOptionsTest
     end
 
     test "Convert deprecated certs, verify with warning" do
+      omit_if_jruby "SSL tests don't work yet"
       with_fake_server(preauth: false) do |server, imap|
         assert_deprecated_warning(/Call Net::IMAP#starttls with keyword/i) do
           imap.starttls(server.config.tls[:ca_file], false)
diff --git a/test/net/imap/test_vanished_data.rb b/test/net/imap/test_vanished_data.rb
index 72e3f53ee..da53e4164 100644
--- a/test/net/imap/test_vanished_data.rb
+++ b/test/net/imap/test_vanished_data.rb
@@ -54,11 +54,13 @@ class VanishedDataTest < Net::IMAP::TestCase
   end
 
   test ".[], mixing args raises ArgumentError" do
-    assert_raise ArgumentError do
-      VanishedData[1, true, uids: "1", earlier: true]
+    pend_if_jruby do
+      assert_raise ArgumentError do
+        VanishedData[1, true, uids: "1", earlier: true]
+      end
+      assert_raise ArgumentError do VanishedData["1234", earlier: true] end
+      assert_raise ArgumentError do VanishedData[nil, true, uids: "1"]  end
     end
-    assert_raise ArgumentError do VanishedData["1234", earlier: true] end
-    assert_raise ArgumentError do VanishedData[nil, true, uids: "1"]  end
   end
 
   test ".[], missing args raises ArgumentError" do