Allow use (with deliberate opt-in) with no security policy enforcement, as on Java >= 24

CI/integration

@@ -131,15 +131,29 @@ int jFeatureVersion = Runtime.version().major();
 
 String vmopts = "-enableassertions:org.postgresql.pljava... -Xcheck:jni";
 
-if ( 17 < jFeatureVersion )
-  vmopts += " -Djava.security.manager=allow";
+vmopts += " --limit-modules=org.postgresql.pljava.internal";
+
+if ( 24 <= jFeatureVersion ) {
+  vmopts += " -Djava.security.manager=disallow"; // JEP 486
+} else if ( 18 <= jFeatureVersion )
+  vmopts += " -Djava.security.manager=allow"; // JEP 411
 
 if ( 23 <= jFeatureVersion )
   vmopts += " --sun-misc-unsafe-memory-access=deny"; // JEP 471
 
 if ( 24 <= jFeatureVersion )
   vmopts += " --illegal-native-access=deny"; // JEP 472
 
+Map<String,String> serverOptions = new HashMap<>(Map.of(
+  "client_min_messages", "info",
+  "pljava.vmoptions", vmopts,
+  "pljava.libjvm_location", libjvm.toString()
+));
+if ( 24 <= jFeatureVersion ) {
+  serverOptions.put("pljava.allow_unenforced", "java,java_tzset");
+  serverOptions.put("pljava.allow_unenforced_udt", "on");
+}
+
 Node n1 = Node.get_new_node("TestNode1");
 
 if ( s_isWindows )
@@ -208,11 +222,7 @@ throws Exception
 
 try (
   AutoCloseable t1 = n1.initialized_cluster(tweaks);
-  AutoCloseable t2 = n1.started_server(Map.of(
-    "client_min_messages", "info",
-    "pljava.vmoptions", vmopts,
-    "pljava.libjvm_location", libjvm.toString()
-  ), tweaks);
+  AutoCloseable t2 = n1.started_server(serverOptions, tweaks);
 )
 {
   int pgMajorVersion;

pljava-api/src/main/java/org/postgresql/pljava/Session.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2004-2022 Tada AB and other contributors, as listed below.
+ * Copyright (c) 2004-2025 Tada AB and other contributors, as listed below.
  *
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the The BSD 3-Clause License
@@ -17,6 +17,8 @@
 import java.sql.Connection;
 import java.sql.SQLException;
 
+import java.util.Properties;
+
 /**
  * A Session brings together some useful methods and data for the current
  * database session. It provides a set of attributes (a
@@ -35,6 +37,21 @@
  */
 public interface Session
 {
+	/**
+	 * Returns an unmodifiable defensive copy of the Java
+	 * {@link System#getProperties() system properties} taken early in PL/Java
+	 * startup before user code has an opportunity to write them.
+	 *<p>
+	 * When PL/Java is running without security policy enforcement, as on stock
+	 * Java 24 and later, using the frozen properties can simplify defensive
+	 * coding against the possibility of arbitrary property modifications.
+	 *
+	 * @return a {@link Properties} object that departs from the API spec by
+	 * throwing {@link UnsupportedOperationException} from any method if the
+	 * properties would otherwise be modified.
+	 */
+	Properties frozenSystemProperties();
+
 	/**
 	 * Adds the specified {@code listener} to the list of listeners that will
 	 * receive savepoint events. An {@link AccessControlContext} saved by this

pljava-api/src/main/java/org/postgresql/pljava/SessionManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2004-2019 Tada AB and other contributors, as listed below.
+ * Copyright (c) 2004-2025 Tada AB and other contributors, as listed below.
  *
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the The BSD 3-Clause License
@@ -23,21 +23,41 @@
  */
 public class SessionManager
 {
-	private static Session s_session;
-
 	/**
 	 * Returns the current session.
 	 */
 	public static Session current()
 	throws SQLException
 	{
-		if(s_session == null)
+		try
+		{
+			return Holder.s_session;
+		}
+		catch ( ExceptionInInitializerError e )
 		{
-			s_session = load(
-				Session.class.getModule().getLayer(), Session.class)
-				.findFirst().orElseThrow(() -> new SQLException(
-					"could not obtain PL/Java Session object"));
+			Throwable c = e.getCause();
+			if ( c instanceof SQLException )
+				throw (SQLException)c;
+			throw e;
+		}
+	}
+
+	private static class Holder
+	{
+		private static final Session s_session;
+
+		static {
+			try
+			{
+				s_session = load(
+					Session.class.getModule().getLayer(), Session.class)
+					.findFirst().orElseThrow(() -> new SQLException(
+						"could not obtain PL/Java Session object"));
+			}
+			catch ( SQLException e )
+			{
+				throw new ExceptionInInitializerError(e);
+			}
 		}
-		return s_session;
 	}
 }

pljava-examples/src/main/java/org/postgresql/pljava/example/annotation/JDBC42_21.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2023 Tada AB and other contributors, as listed below.
+ * Copyright (c) 2018-2025 Tada AB and other contributors, as listed below.
  *
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the The BSD 3-Clause License
@@ -11,6 +11,10 @@
  */
 package org.postgresql.pljava.example.annotation;
 
+import java.sql.SQLException;
+
+import org.postgresql.pljava.SessionManager;
+
 import org.postgresql.pljava.annotation.Function;
 import org.postgresql.pljava.annotation.SQLAction;
 
@@ -133,9 +137,10 @@ public class JDBC42_21
 	 * recent as the argument ('1.6', '1.7', '1.8', '9', '10', '11', ...).
 	 */
 	@Function(schema="javatest", provides="javaSpecificationGE")
-	public static boolean javaSpecificationGE(String want)
+	public static boolean javaSpecificationGE(String want) throws SQLException
 	{
-		String got = System.getProperty("java.specification.version");
+		String got = SessionManager.current().frozenSystemProperties()
+			.getProperty("java.specification.version");
 		if ( want.startsWith("1.") )
 			want = want.substring(2);
 		if ( got.startsWith("1.") )

pljava-examples/src/main/java/org/postgresql/pljava/example/annotation/Modules.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright (c) 2025 Tada AB and other contributors, as listed below.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the The BSD 3-Clause License
+ * which accompanies this distribution, and is available at
+ * http://opensource.org/licenses/BSD-3-Clause
+ *
+ * Contributors:
+ *   Chapman Flack
+ */
+package org.postgresql.pljava.example.annotation;
+
+import java.lang.module.ModuleDescriptor;
+
+import java.sql.ResultSet;
+import java.sql.SQLException;
+
+import java.util.Iterator;
+import java.util.Objects;
+
+import java.util.stream.Stream;
+
+import org.postgresql.pljava.ResultSetProvider;
+import org.postgresql.pljava.annotation.Function;
+import static org.postgresql.pljava.annotation.Function.Effects.STABLE;
+
+/**
+ * Example code to support querying for the modules in Java's boot layer.
+ */
+public class Modules implements ResultSetProvider.Large {
+	/**
+	 * Returns information on the named modules in Java's boot module layer.
+	 */
+	@Function(
+		effects = STABLE,
+		out = {
+			                   "name  pg_catalog.text",
+			"any_unqualified_exports  boolean",
+			  "any_unqualified_opens  boolean"
+		}
+	)
+	public static ResultSetProvider java_modules()
+	{
+		return new Modules(
+			ModuleLayer.boot().modules().stream().map(Module::getDescriptor)
+				.filter(Objects::nonNull));
+	}
+
+	private final Iterator<ModuleDescriptor> iterator;
+	private final Runnable closer;
+
+	private Modules(Stream<ModuleDescriptor> s)
+	{
+		iterator = s.iterator();
+		closer = s::close;
+	}
+
+	@Override
+	public boolean assignRowValues(ResultSet receiver, long currentRow)
+	throws SQLException
+	{
+		if ( ! iterator.hasNext() )
+			return false;
+
+		ModuleDescriptor md = iterator.next();
+
+		receiver.updateString(1, md.name());
+
+		receiver.updateBoolean(2,
+			md.exports().stream().anyMatch(e -> ! e.isQualified()));
+
+		receiver.updateBoolean(3,
+			md.isOpen() ||
+			md.opens().stream().anyMatch(o -> ! o.isQualified()));
+
+		return true;
+	}
+
+	@Override
+	public void close()
+	{
+		closer.run();
+	}
+}

pljava-examples/src/main/java/org/postgresql/pljava/example/annotation/PassXML.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2024 Tada AB and other contributors, as listed below.
+ * Copyright (c) 2018-2025 Tada AB and other contributors, as listed below.
  *
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the The BSD 3-Clause License
@@ -67,6 +67,7 @@
 
 import org.postgresql.pljava.Adjusting;
 import static org.postgresql.pljava.Adjusting.XML.setFirstSupported;
+import org.postgresql.pljava.SessionManager;
 import org.postgresql.pljava.annotation.Function;
 import org.postgresql.pljava.annotation.MappedUDT;
 import org.postgresql.pljava.annotation.SQLAction;
@@ -643,7 +644,8 @@ public static SQLXML transformXML(
 			 */
 			if ( rlt instanceof StreamResult )
 				t.setOutputProperty(ENCODING,
-					System.getProperty("org.postgresql.server.encoding"));
+					SessionManager.current().frozenSystemProperties()
+					.getProperty("org.postgresql.server.encoding"));
 			else if ( Boolean.TRUE.equals(indent) )
 				logMessage("WARNING",
 					"indent requested, but howout specifies a non-stream " +
@@ -712,7 +714,8 @@ private static SQLXML echoSQLXML(SQLXML sx, int howin, int howout)
 			 */
 			if ( howout < 5 )
 				t.setOutputProperty(ENCODING,
-					System.getProperty("org.postgresql.server.encoding"));
+					SessionManager.current().frozenSystemProperties()
+					.getProperty("org.postgresql.server.encoding"));
 			t.transform(src, rlt);
 		}
 		catch ( TransformerException te )

pljava-examples/src/main/java/org/postgresql/pljava/example/annotation/SPIActions.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2004-2020 Tada AB and other contributors, as listed below.
+ * Copyright (c) 2004-2025 Tada AB and other contributors, as listed below.
  *
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the The BSD 3-Clause License
@@ -127,20 +127,26 @@ public static String getTimeAsString() throws SQLException {
 		}
 	}
 
-	static void log(String msg) {
+	static void log(String msg) throws SQLException {
 		// GCJ has a somewhat serious bug (reported)
 		//
-		if ("GNU libgcj".equals(System.getProperty("java.vm.name"))) {
+		if ("GNU libgcj"
+				.equals(
+					SessionManager.current().frozenSystemProperties()
+						.getProperty("java.vm.name"))) {
 			System.out.print("INFO: ");
 			System.out.println(msg);
 		} else
 			Logger.getAnonymousLogger().info(msg);
 	}
 
-	static void warn(String msg) {
+	static void warn(String msg) throws SQLException {
 		// GCJ has a somewhat serious bug (reported)
 		//
-		if ("GNU libgcj".equals(System.getProperty("java.vm.name"))) {
+		if ("GNU libgcj"
+				.equals(
+					SessionManager.current().frozenSystemProperties()
+						.getProperty("java.vm.name"))) {
 			System.out.print("WARNING: ");
 			System.out.println(msg);
 		} else