JEP draft: Deprecate Memory-Access Methods in sun.misc.Unsafe for Removal

AuthorRon Pressler & Alex Buckley
OwnerRon Pressler
TypeFeature
ScopeJDK
StatusSubmitted
Reviewed byAlan Bateman, Brian Goetz, Maurizio Cimadamore, Paul Sandoz
Created2024/01/05 15:46
Updated2024/02/27 15:34
Issue8323072

Summary

Deprecate the memory-access methods in sun.misc.Unsafe for removal in a future release. These unsupported methods have had supported replacements since JDK 9 (for accessing on-heap memory) and JDK 22 (for accessing off-heap memory). Library developers are strongly encouraged to migrate from sun.misc.Unsafe to the supported replacements, so that applications can migrate smoothly to modern JDKs.

Goals

Non-Goals

Motivation

The sun.misc.Unsafe class was introduced in 2002 as a way for Java classes in the JDK to perform low-level operations. Most of its methods — 79 out of 87 — are for accessing memory, either in the JVM's garbage-collected heap or in "off-heap" memory not controlled by the JVM. As the name of the class suggests, these memory-access methods are unsafe — they can lead to undefined behavior, including JVM crashes — so they were not exposed as a standard API. They were neither envisaged for use by a broad range of clients nor intended to be permanent. Rather, they were introduced with the assumption that they were exclusively for use within the JDK, and that callers within the JDK would perform exhaustive safety checks before using them, and that safe standard APIs for this functionality would eventually be added to the Java Platform.

However, with no way in 2002 to prevent sun.misc.Unsafe from being used outside the JDK, its memory-access methods became a "Swiss army knife" for library developers who wanted more power and performance than standard APIs could offer. For example, sun.misc.Unsafe::compareAndSwap can perform a CAS (compare-and-swap) operation on a field without the overhead of java.util.concurrent.atomic, while sun.misc.Unsafe::setMemory can manipulate off-heap memory without the 2GB limitation of java.nio.ByteBuffer. Where libraries such as Apache Hadoop and Cassandra do rely on java.nio.ByteBuffer to manipulate off-heap memory, sun.misc.Unsafe::invokeCleaner can improve efficiency by deallocating off-heap memory promptly.

Unfortunately, not all libraries are diligent at performing safety checks before calling the memory-access methods, so there is a risk of failures and crashes in applications. Some uses of the methods are unnecessary, driven by the ease of copy-and-paste from online forums. Other uses of the methods may cause the JVM to disable optimizations, resulting in worse performance for callers than if they had used ordinary Java arrays. Nevertheless, because use of the memory-access methods is so widespread, sun.misc.Unsafe was not encapsulated alongside other low-level APIs in JDK 9. It remains available "out of the box" in JDK 22, pending the availability of safe supported alternatives.

Over the past several years, we have introduced two standard APIs that are safe and performant replacements for the memory-access methods in sun.misc.Unsafe:

These standard APIs guarantee no undefined behavior, promise long-term stability, and have high-quality integration with the tooling and documentation of the Java Platform (examples of their use are given below). Given the availability of these APIs, it is now appropriate to deprecate and eventually remove the memory-access methods in sun.misc.Unsafe.

The removal of the memory-access methods in sun.misc.Unsafe is part of a coordinated effort to provide integrity by default in the Java Platform. Other initiatives include restrictions on JNI and on the dynamic loading of agents. These efforts are necessary to make the Java Platform more secure and performant. They also mean that application developers will no longer get stuck on older JDKs because of libraries that break on newer JDKs when unsupported APIs are changed.

Description

The memory-access methods of sun.misc.Unsafe can be divided into three categories:

We will proceed as follows, where each phase takes place in a separate successive JDK feature release:

  1. Deprecate all memory-access methods — on-heap, off-heap, and bimodal — for removal. This will cause compile-time deprecation warnings for code that refers to the methods, alerting library developers to their forthcoming removal. A new command-line option, described below, will allow users to receive runtime warnings when the methods are used.

    Distinct from deprecation warnings, javac has given warnings about the use of sun.misc.Unsafe since 2006:

    warning: Unsafe is internal proprietary API and may be removed in a future release

    These warnings will continue to appear, and cannot be suppressed.

  2. Issue a warning at run time, as detailed below, when a memory-access method is used (including through reflection). This will alert application developers and users to the forthcoming removal of the methods, and the need to upgrade libraries in future.

  3. Throw an exception when a memory-access method is used (including through reflection). This will further alert application developers and users to the imminent removal of the methods.

  4. Remove on-heap methods. These methods will be removed first because they have had standard replacements since JDK 9 in 2017.

  5. Remove off-heap and bimodal methods. These methods will be removed later because they have had standard replacements only since JDK 22 in 2023.

We plan to implement the run-time warnings of phase 2 in or before JDK 25.

We plan to implement the removal of methods in phases 4 and 5 in JDK 26 or later. We may perform phases 4 and 5 simultaneously if appropriate when the time arrives.

Allowing the use of memory-access methods in sun.misc.Unsafe

The vast majority of Java developers do not use sun.misc.Unsafe explicitly in their own code. However, many applications depend, directly or indirectly, on libraries that use the memory-access methods of sun.misc.Unsafe. Starting in JDK 23, application developers can assess how the methods' deprecation and removal will affect libraries on the class path by running with a new command line option, --sun-misc-unsafe-memory-access={allow|warn|debug|deny} (which is similar, in spirit and in form, to the --illegal-access flag introduced by JEP 261):

The warning from warn is as follows:

WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::setMemory has been called by com.foo.bar.Server (file:/tmp/foobarserver/thing.jar)
WARNING: Please consider reporting this to the maintainers of com.foo.bar.Server
WARNING: sun.misc.Unsafe::setMemory will be removed in a future release

The default value of --sun-misc-unsafe-memory-access will change from release to release as we proceed with the deprecation and removal of memory-access methods:

The following tools in the JDK help advanced developers to understand how their application or library uses deprecated methods in sun.misc.Unsafe:

sun.misc.Unsafe memory-access methods and their replacements

On-heap methods

These methods exist to obtain offsets (or scale) which are then used with the bimodal methods below to read and write fields or array elements. This usage is replaced by VarHandle and MemorySegment::ofArray.

In rare circumstances, these methods are used on their own to examine and manipulate the physical layout of an object in memory (see examples here). There is no supported replacement for this use case; see below for further discussion.

The first three methods above were already deprecated in JDK 18.

In addition, the following fields that are associated with these methods are deprecated for removal:

Off-heap methods

FFM downcalls, referenced below, are described here.

Bimodal memory-access methods

Migration examples

On-heap memory access

Suppose class Foo has an int field that we wish to atomically double. With sun.misc.Unsafe, the code is, assuming UNSAFE is a static field that refers to a sun.misc.Unsafe object:

class Foo {
    private int x;
    
    public boolean tryAtomicallyDoubleX() {
        int oldValue = x;
        return UNSAFE.compareAndSwapInt(this, X_OFFSET, oldValue, oldValue*2);
    }
    
    private static final long X_OFFSET;
    static {
        try {
            X_OFFSET = UNSAFE.objectFieldOffset(Foo.class.getDeclaredField("x"));
        } catch (Exception ex) { throw new AssertionError(ex); }
    }
}

With the standard VarHandle API, the code is:

class Foo {
    private int x;
    
    public boolean tryAtomicallyDoubleX() {
        int oldValue = x;
        return X_VH.compareAndSet(this, oldValue, oldValue*2);
    }
    
    private static final VarHandle X_VH;
    static {
        try {
            X_VH = MethodHandles.lookup().findVarHandle(Foo.class, "x", int.class);
        } catch (Exception ex) { throw new AssertionError(ex); }
    }
}

Here is a class that uses sun.misc.Unsafe to perform a volatile write of an array element:

class Foo {
    private int[] a = new int[10];
    
    public void setVolatile(int index, int value) {
        if (index < 0 || index >= a.length)
            throw new ArrayIndexOutOfBoundsException(index);
        UNSAFE.putIntVolatile(a, ARRAY_BASE + ARRAY_SCALE*index, value);
    }
    
    private static final int ARRAY_BASE = UNSAFE.arrayBaseOffset(int[].class);
    private static final int ARRAY_SCALE = UNSAFE.arrayIndexScale(int[].class);
}

With VarHandle, the code is:

class Foo {
    private int[] a = new int[10];
    
    public void setVolatile(int index, int value) {
        AVH.setVolatile(a, index, value);
    }
    
    private static final VarHandle AVH = MethodHandles.arrayElementVarHandle(int[].class);
}

Off-heap memory access

Here is a class that uses sun.misc.Unsafe to allocate an off-heap buffer and perform three operations: a volatile write of an int, a bulk initialization of a subset of the buffer, and a copy of the buffer data into a Java int array:

class OffHeapIntBuffer {
    private final long size;
    private long bufferPtr;
    
    public OffHeapIntBuffer(long size) {
        this.size = size;
        this.bufferPtr = UNSAFE.allocateMemory(size*ARRAY_SCALE);
    }
    
    public void deallocate() {
        if (bufferPtr == 0) return;
        UNSAFE.freeMemory(bufferPtr);
        bufferPtr = 0;
    }
    
    public void setVolatile(long index, int value) {
        checkBounds(index);
        UNSAFE.putIntVolatile(null, bufferPtr + ARRAY_SCALE*index, value);
    }
    
    public void initialize(long start, long n) {
        checkBounds(start);
        checkBounds(start + n-1);
        UNSAFE.setMemory(bufferPtr + start*ARRAY_SCALE, n*ARRAY_SCALE, 0);
    }
    
    public int[] copyToNewArray(long start, int n) {
        checkBounds(start);
        checkBounds(start + n-1);
        int[] a = new int[n];
        UNSAFE.copyMemory(null, bufferPtr + start*ARRAY_SCALE, a, ARRAY_BASE, n*ARRAY_SCALE);
        return a;
    }
    
    private boolean checkBounds(long index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException(index);
        return true;
    }
    
    private static final int ARRAY_BASE = UNSAFE.arrayBaseOffset(int[].class);
    private static final int ARRAY_SCALE = UNSAFE.arrayIndexScale(int[].class);
}

With the standard Arena and MemorySegment APIs, the code is:

class OffHeapIntBuffer {
    private static final VarHandle ELEM_VH = ValueLayout.JAVA_INT.arrayElementVarHandle();

    private final Arena arena;
    private final MemorySegment buffer;
    
    public OffHeapIntBuffer(long size) {
        this.arena  = Arena.ofShared();
        this.buffer = arena.allocate(ValueLayout.JAVA_INT, size);
    }
    
    public void deallocate() {
        arena.close();
    }
    
    public void setVolatile(long index, int value) {
        ELEM_VH.setVolatile(buffer, 0L, index, value);
    }
    
    public void initialize(long start, long n) {
        buffer.asSlice(ValueLayout.JAVA_INT.byteSize() * start,
                       ValueLayout.JAVA_INT.byteSize() * n)
              .fill((byte) 0);
    }
    
    public int[] copyToNewArray(long start, int n) {
        return buffer.asSlice(ValueLayout.JAVA_INT.byteSize() * start,
                              ValueLayout.JAVA_INT.byteSize() * n)
                     .toArray(ValueLayout.JAVA_INT);
    }
}

Risks and Assumptions

Future Work

After deprecating the 79 memory-access methods for removal, sun.misc.Unsafe will contain only three methods that are not deprecated: