Initial import: Backport of jdk-sandbox JSON API to JDK 24

Backport of the experimental JSON API from jdk-sandbox commit d22dc2ba8
to work with JDK 24. This is a simplified version with optimizations and
value-based features removed for compatibility.

Changes from original:
- Removed StableValue optimizations
- Removed value-based class annotations
- Basic implementation without performance optimizations
- Compatible with JDK 24 instead of future JDK versions

Provides a functional JSON parser and API including:
- JsonValue hierarchy (JsonObject, JsonArray, JsonString, etc.)
- JSON parsing with JsonParser
- Basic immutable value objects

Licensed under GNU General Public License version 2 with Classpath exception.

Author: simbo1905

.gitignore

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+target/
+
+.idea/

LICENSE

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+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>jdk-sandbox</groupId>
+  <artifactId>json-experimental</artifactId>
+  <version>1.0-SNAPSHOT</version>
+  <packaging>jar</packaging>
+
+  <name>JSON Experimental</name>
+  <description>Experimental JSON API and internal dependencies</description>
+
+  <licenses>
+    <license>
+      <name>GNU General Public License, version 2, with the Classpath Exception</name>
+      <url>https://www.gnu.org/licenses/old-licenses/gpl-2.0.html</url>
+      <distribution>repo</distribution>
+    </license>
+  </licenses>
+
+  <properties>
+    <maven.compiler.source>24</maven.compiler.source>
+    <maven.compiler.target>24</maven.compiler.target>
+  </properties>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <version>3.8.1</version>
+        <configuration>
+          <source>24</source>
+          <target>24</target>
+        </configuration>
+      </plugin>
+    </plugins>
+  </build>
+</project>

Towards a JSON API for the JDK.pdf

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+package jdk.sandbox;
+
+import java.util.function.Supplier;
+
+/**
+ * Mimics JDK's StableValue using double-checked locking pattern
+ * for thread-safe lazy initialization.
+ */
+public class StableValue<T> {
+    private volatile T value;
+    private final Object lock = new Object();
+    
+    private StableValue() {}
+    
+    public static <T> StableValue<T> of() {
+        return new StableValue<>();
+    }
+    
+    public T get() {
+        return value;
+    }
+    
+    public void set(T value) {
+        if (this.value == null) {
+            synchronized (lock) {
+                if (this.value == null) {
+                    this.value = value;
+                }
+            }
+        }
+    }
+    
+    public T orElse(T defaultValue) {
+        T result = value;
+        return result != null ? result : defaultValue;
+    }
+    
+    public T orElseSet(Supplier<T> supplier) {
+        T result = value;
+        if (result == null) {
+            synchronized (lock) {
+                result = value;
+                if (result == null) {
+                    value = result = supplier.get();
+                }
+            }
+        }
+        return result;
+    }
+    
+    public void setOrThrow(T newValue) {
+        if (value != null) {
+            throw new IllegalStateException("Value already set");
+        }
+        synchronized (lock) {
+            if (value != null) {
+                throw new IllegalStateException("Value already set");
+            }
+            value = newValue;
+        }
+    }
+    
+    public static <T> Supplier<T> supplier(Supplier<T> supplier) {
+        return new Supplier<T>() {
+            private volatile T cached;
+            private final Object supplierLock = new Object();
+            
+            @Override
+            public T get() {
+                T result = cached;
+                if (result == null) {
+                    synchronized (supplierLock) {
+                        result = cached;
+                        if (result == null) {
+                            cached = result = supplier.get();
+                        }
+                    }
+                }
+                return result;
+            }
+        };
+    }
+}

pom.xml

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+package jdk.sandbox.demo;
+
+import jdk.sandbox.java.util.json.Json;
+import jdk.sandbox.java.util.json.JsonObject;
+import jdk.sandbox.java.util.json.JsonString;
+import jdk.sandbox.java.util.json.JsonNumber;
+import java.util.Map;
+
+public class JsonDemo {
+    public static void main(String[] args) {
+        // Create a JSON object using the factory method
+        JsonObject jsonObject = JsonObject.of(Map.of(
+            "name", JsonString.of("John"),
+            "age", JsonNumber.of(30)
+        ));
+
+        System.out.println(jsonObject.toString());
+        
+        // Parse JSON string
+        String jsonStr = "{\"name\":\"Jane\",\"age\":25}";
+        var parsed = Json.parse(jsonStr);
+        System.out.println("Parsed: " + parsed);
+    }
+}

src/main/java/jdk/sandbox/StableValue.java

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+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package jdk.sandbox.internal.util.json;
+
+import java.util.Collections;
+import java.util.List;
+import jdk.sandbox.java.util.json.JsonArray;
+import jdk.sandbox.java.util.json.JsonValue;
+
+
+/**
+ * JsonArray implementation class
+ */
+public final class JsonArrayImpl implements JsonArray {
+
+    private final List<JsonValue> theValues;
+
+    public JsonArrayImpl(List<JsonValue> from) {
+        theValues = from;
+    }
+
+    @Override
+    public List<JsonValue> values() {
+        return Collections.unmodifiableList(theValues);
+    }
+
+    @Override
+    public String toString() {
+        var s = new StringBuilder("[");
+        for (JsonValue v: values()) {
+            s.append(v.toString()).append(",");
+        }
+        if (!values().isEmpty()) {
+            s.setLength(s.length() - 1); // trim final comma
+        }
+        return s.append("]").toString();
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        return o instanceof JsonArray oja &&
+                values().equals(oja.values());
+    }
+
+    @Override
+    public int hashCode() {
+        return values().hashCode();
+    }
+}

src/main/java/jdk/sandbox/demo/JsonDemo.java

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+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package jdk.sandbox.internal.util.json;
+
+import jdk.sandbox.java.util.json.JsonBoolean;
+
+
+/**
+ * JsonBoolean implementation class
+ */
+public final class JsonBooleanImpl implements JsonBoolean {
+
+    private final Boolean theBoolean;
+
+    public static final JsonBooleanImpl TRUE = new JsonBooleanImpl(true);
+    public static final JsonBooleanImpl FALSE = new JsonBooleanImpl(false);
+
+    private JsonBooleanImpl(Boolean bool) {
+        theBoolean = bool;
+    }
+
+    @Override
+    public boolean value() {
+        return theBoolean;
+    }
+
+    @Override
+    public String toString() {
+        return String.valueOf(value());
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        return o instanceof JsonBoolean ojb && value() == ojb.value();
+    }
+
+    @Override
+    public int hashCode() {
+        return Boolean.hashCode(value());
+    }
+}

src/main/java/jdk/sandbox/internal/util/json/JsonArrayImpl.java

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+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package jdk.sandbox.internal.util.json;
+
+import jdk.sandbox.java.util.json.JsonNull;
+
+
+/**
+ * JsonNull implementation class
+ */
+public final class JsonNullImpl implements JsonNull {
+
+    public static final JsonNullImpl NULL = new JsonNullImpl();
+    private static final String VALUE = "null";
+    private static final int HASH = VALUE.hashCode();
+
+    private JsonNullImpl() {}
+
+    @Override
+    public String toString() {
+        return VALUE;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        return obj instanceof JsonNull;
+    }
+
+    @Override
+    public int hashCode() {
+        return HASH;
+    }
+}