sun.misc.Unsafe源代码

﹏ヽ暗。殇╰゛Y 2022-08-07 08:47 134阅读 0赞

Unsafe类提供了硬件级别的原子操作，Java无法直接访问到操作系统底层（如系统硬件等)，为此Java使用native方法来扩展Java程序的功能。Java并发包(java.util.concurrent)中大量使用了Unsafe类提供的CAS方法。Unsafe类还提供了阻塞和唤醒线程的方法，以及volatile read/write操作等。

下面是sun.misc.Unsafe.java类的部分源码(jdk1.8)，以便阅读JUC源码的时候提供参照之用：

/**
 * A collection of methods for performing low-level, unsafe operations.
 * Although the class and all methods are public, use of this class is
 * limited because only trusted code can obtain instances of it.
 *
 * @author John R. Rose
 * @see #getUnsafe
 */
 
 public final class Unsafe {
 
 private static native void registerNatives();
 static {
 registerNatives();
 sun.reflect.Reflection.registerMethodsToFilter(Unsafe.class, "getUnsafe");
 }
 
 private Unsafe() {}
 
 private static final Unsafe theUnsafe = new Unsafe();
 
 /**
 * Provides the caller with the capability of performing unsafe
 * operations.
 *
 * The returned <code>Unsafe</code> object should be carefully guarded
 * by the caller, since it can be used to read and write data at arbitrary
 * memory addresses. It must never be passed to untrusted code.
 *
 * Most methods in this class are very low-level, and correspond to a
 * small number of hardware instructions (on typical machines). Compilers
 * are encouraged to optimize these methods accordingly.
 *
 * Here is a suggested idiom for using unsafe operations:
 *
 * <blockquote><pre>
 * class MyTrustedClass {
 * private static final Unsafe unsafe = Unsafe.getUnsafe();
 * ...
 * private long myCountAddress = ...;
 * public int getCount() { return unsafe.getByte(myCountAddress); }
 * }
 * </pre></blockquote>
 *
 * (It may assist compilers to make the local variable be
 * <code>final</code>.)
 *
 * @exception SecurityException if a security manager exists and its
 * <code>checkPropertiesAccess</code> method doesn't allow
 * access to the system properties.
 */
 @CallerSensitive
 public static Unsafe getUnsafe() {
 Class<?> caller = Reflection.getCallerClass();
 if (!VM.isSystemDomainLoader(caller.getClassLoader()))
 throw new SecurityException("Unsafe");
 return theUnsafe;
 }
 
 /// peek and poke operations
 /// (compilers should optimize these to memory ops)
 
 // These work on object fields in the Java heap.
 // They will not work on elements of packed arrays.
 
 /**
 * Fetches a value from a given Java variable.
 * More specifically, fetches a field or array element within the given
 * object <code>o</code> at the given offset, or (if <code>o</code> is
 * null) from the memory address whose numerical value is the given
 * offset.
 * 
 * The results are undefined unless one of the following cases is true:
 * <ul>
 * <li>The offset was obtained from {@link #objectFieldOffset} on
 * the {@link java.lang.reflect.Field} of some Java field and the object
 * referred to by <code>o</code> is of a class compatible with that
 * field's class.
 *
 * <li>The offset and object reference <code>o</code> (either null or
 * non-null) were both obtained via {@link #staticFieldOffset}
 * and {@link #staticFieldBase} (respectively) from the
 * reflective {@link Field} representation of some Java field.
 *
 * <li>The object referred to by <code>o</code> is an array, and the offset
 * is an integer of the form <code>B+N*S</code>, where <code>N</code> is
 * a valid index into the array, and <code>B</code> and <code>S</code> are
 * the values obtained by {@link #arrayBaseOffset} and {@link
 * #arrayIndexScale} (respectively) from the array's class. The value
 * referred to is the <code>N</code>th element of the array.
 *
 * </ul>
 * 
 * If one of the above cases is true, the call references a specific Java
 * variable (field or array element). However, the results are undefined
 * if that variable is not in fact of the type returned by this method.
 * 
 * This method refers to a variable by means of two parameters, and so
 * it provides (in effect) a double-register addressing mode
 * for Java variables. When the object reference is null, this method
 * uses its offset as an absolute address. This is similar in operation
 * to methods such as {@link #getInt(long)}, which provide (in effect) a
 * single-register addressing mode for non-Java variables.
 * However, because Java variables may have a different layout in memory
 * from non-Java variables, programmers should not assume that these
 * two addressing modes are ever equivalent. Also, programmers should
 * remember that offsets from the double-register addressing mode cannot
 * be portably confused with longs used in the single-register addressing
 * mode.
 *
 * @param o Java heap object in which the variable resides, if any, else
 * null
 * @param offset indication of where the variable resides in a Java heap
 * object, if any, else a memory address locating the variable
 * statically
 * @return the value fetched from the indicated Java variable
 * @throws RuntimeException No defined exceptions are thrown, not even
 * {@link NullPointerException}
 */
 public native int getInt(Object o, long offset);
 
 /**
 * Stores a value into a given Java variable.
 * 
 * The first two parameters are interpreted exactly as with
 * {@link #getInt(Object, long)} to refer to a specific
 * Java variable (field or array element). The given value
 * is stored into that variable.
 * 
 * The variable must be of the same type as the method
 * parameter <code>x</code>.
 *
 * @param o Java heap object in which the variable resides, if any, else
 * null
 * @param offset indication of where the variable resides in a Java heap
 * object, if any, else a memory address locating the variable
 * statically
 * @param x the value to store into the indicated Java variable
 * @throws RuntimeException No defined exceptions are thrown, not even
 * {@link NullPointerException}
 */
 public native void putInt(Object o, long offset, int x);
 
 /**
 * Fetches a reference value from a given Java variable.
 * @see #getInt(Object, long)
 */
 public native Object getObject(Object o, long offset);
 
 /**
 * Stores a reference value into a given Java variable.
 * 
 * Unless the reference <code>x</code> being stored is either null
 * or matches the field type, the results are undefined.
 * If the reference <code>o</code> is non-null, car marks or
 * other store barriers for that object (if the VM requires them)
 * are updated.
 * @see #putInt(Object, int, int)
 */
 public native void putObject(Object o, long offset, Object x);
 
 
 // These work on values in the C heap.
 
 /**
 * Fetches a value from a given memory address. If the address is zero, or
 * does not point into a block obtained from {@link #allocateMemory}, the
 * results are undefined.
 *
 * @see #allocateMemory
 */
 public native byte getByte(long address);
 
 /**
 * Stores a value into a given memory address. If the address is zero, or
 * does not point into a block obtained from {@link #allocateMemory}, the
 * results are undefined.
 *
 * @see #getByte(long)
 */
 public native void putByte(long address, byte x);
 
 
 /**
 * Fetches a native pointer from a given memory address. If the address is
 * zero, or does not point into a block obtained from {@link
 * #allocateMemory}, the results are undefined.
 *
 * If the native pointer is less than 64 bits wide, it is extended as
 * an unsigned number to a Java long. The pointer may be indexed by any
 * given byte offset, simply by adding that offset (as a simple integer) to
 * the long representing the pointer. The number of bytes actually read
 * from the target address maybe determined by consulting {@link
 * #addressSize}.
 *
 * @see #allocateMemory
 */
 public native long getAddress(long address);
 
 /**
 * Stores a native pointer into a given memory address. If the address is
 * zero, or does not point into a block obtained from {@link
 * #allocateMemory}, the results are undefined.
 *
 * The number of bytes actually written at the target address maybe
 * determined by consulting {@link #addressSize}.
 *
 * @see #getAddress(long)
 */
 public native void putAddress(long address, long x);
 
 /// wrappers for malloc, realloc, free:
 
 /**
 * Allocates a new block of native memory, of the given size in bytes. The
 * contents of the memory are uninitialized; they will generally be
 * garbage. The resulting native pointer will never be zero, and will be
 * aligned for all value types. Dispose of this memory by calling {@link
 * #freeMemory}, or resize it with {@link #reallocateMemory}.
 *
 * @throws IllegalArgumentException if the size is negative or too large
 * for the native size_t type
 *
 * @throws OutOfMemoryError if the allocation is refused by the system
 *
 * @see #getByte(long)
 * @see #putByte(long, byte)
 */
 public native long allocateMemory(long bytes);
 
 /**
 * Resizes a new block of native memory, to the given size in bytes. The
 * contents of the new block past the size of the old block are
 * uninitialized; they will generally be garbage. The resulting native
 * pointer will be zero if and only if the requested size is zero. The
 * resulting native pointer will be aligned for all value types. Dispose
 * of this memory by calling {@link #freeMemory}, or resize it with {@link
 * #reallocateMemory}. The address passed to this method may be null, in
 * which case an allocation will be performed.
 *
 * @throws IllegalArgumentException if the size is negative or too large
 * for the native size_t type
 *
 * @throws OutOfMemoryError if the allocation is refused by the system
 *
 * @see #allocateMemory
 */
 public native long reallocateMemory(long address, long bytes);
 
 /**
 * Sets all bytes in a given block of memory to a fixed value
 * (usually zero).
 *
 * This method determines a block's base address by means of two parameters,
 * and so it provides (in effect) a double-register addressing mode,
 * as discussed in {@link #getInt(Object,long)}. When the object reference is null,
 * the offset supplies an absolute base address.
 *
 * The stores are in coherent (atomic) units of a size determined
 * by the address and length parameters. If the effective address and
 * length are all even modulo 8, the stores take place in 'long' units.
 * If the effective address and length are (resp.) even modulo 4 or 2,
 * the stores take place in units of 'int' or 'short'.
 *
 * @since 1.7
 */
 public native void setMemory(Object o, long offset, long bytes, byte value);
 
 /**
 * Sets all bytes in a given block of memory to a fixed value
 * (usually zero). This provides a single-register addressing mode,
 * as discussed in {@link #getInt(Object,long)}.
 *
 * Equivalent to <code>setMemory(null, address, bytes, value)</code>.
 */
 public void setMemory(long address, long bytes, byte value) {
 setMemory(null, address, bytes, value);
 }
 
 /**
 * Sets all bytes in a given block of memory to a copy of another
 * block.
 *
 * This method determines each block's base address by means of two parameters,
 * and so it provides (in effect) a double-register addressing mode,
 * as discussed in {@link #getInt(Object,long)}. When the object reference is null,
 * the offset supplies an absolute base address.
 *
 * The transfers are in coherent (atomic) units of a size determined
 * by the address and length parameters. If the effective addresses and
 * length are all even modulo 8, the transfer takes place in 'long' units.
 * If the effective addresses and length are (resp.) even modulo 4 or 2,
 * the transfer takes place in units of 'int' or 'short'.
 *
 * @since 1.7
 */
 public native void copyMemory(Object srcBase, long srcOffset,
 Object destBase, long destOffset,
 long bytes);
 /**
 * Sets all bytes in a given block of memory to a copy of another
 * block. This provides a single-register addressing mode,
 * as discussed in {@link #getInt(Object,long)}.
 *
 * Equivalent to <code>copyMemory(null, srcAddress, null, destAddress, bytes)</code>.
 */
 public void copyMemory(long srcAddress, long destAddress, long bytes) {
 copyMemory(null, srcAddress, null, destAddress, bytes);
 }
 
 /**
 * Disposes of a block of native memory, as obtained from {@link
 * #allocateMemory} or {@link #reallocateMemory}. The address passed to
 * this method may be null, in which case no action is taken.
 *
 * @see #allocateMemory
 */
 public native void freeMemory(long address);
 
 
 /**
 * Report the location of a given field in the storage allocation of its
 * class. Do not expect to perform any sort of arithmetic on this offset;
 * it is just a cookie which is passed to the unsafe heap memory accessors.
 *
 * Any given field will always have the same offset and base, and no
 * two distinct fields of the same class will ever have the same offset
 * and base.
 *
 * As of 1.4.1, offsets for fields are represented as long values,
 * although the Sun JVM does not use the most significant 32 bits.
 * However, JVM implementations which store static fields at absolute
 * addresses can use long offsets and null base pointers to express
 * the field locations in a form usable by {@link #getInt(Object,long)}.
 * Therefore, code which will be ported to such JVMs on 64-bit platforms
 * must preserve all bits of static field offsets.
 * @see #getInt(Object, long)
 */
 public native long staticFieldOffset(Field f);
 
 /**
 * Report the location of a given static field, in conjunction with {@link
 * #staticFieldBase}.
 * Do not expect to perform any sort of arithmetic on this offset;
 * it is just a cookie which is passed to the unsafe heap memory accessors.
 *
 * Any given field will always have the same offset, and no two distinct
 * fields of the same class will ever have the same offset.
 *
 * As of 1.4.1, offsets for fields are represented as long values,
 * although the Sun JVM does not use the most significant 32 bits.
 * It is hard to imagine a JVM technology which needs more than
 * a few bits to encode an offset within a non-array object,
 * However, for consistency with other methods in this class,
 * this method reports its result as a long value.
 * @see #getInt(Object, long)
 */
 public native long objectFieldOffset(Field f);
 
 /**
 * Report the location of a given static field, in conjunction with {@link
 * #staticFieldOffset}.
 * Fetch the base "Object", if any, with which static fields of the
 * given class can be accessed via methods like {@link #getInt(Object,
 * long)}. This value may be null. This value may refer to an object
 * which is a "cookie", not guaranteed to be a real Object, and it should
 * not be used in any way except as argument to the get and put routines in
 * this class.
 */
 public native Object staticFieldBase(Field f);
 
 /**
 * Detect if the given class may need to be initialized. This is often
 * needed in conjunction with obtaining the static field base of a
 * class.
 * @return false only if a call to {@code ensureClassInitialized} would have no effect
 */
 public native boolean shouldBeInitialized(Class<?> c);
 
 /**
 * Ensure the given class has been initialized. This is often
 * needed in conjunction with obtaining the static field base of a
 * class.
 */
 public native void ensureClassInitialized(Class<?> c);
 
 /**
 * Report the offset of the first element in the storage allocation of a
 * given array class. If {@link #arrayIndexScale} returns a non-zero value
 * for the same class, you may use that scale factor, together with this
 * base offset, to form new offsets to access elements of arrays of the
 * given class.
 *
 * @see #getInt(Object, long)
 * @see #putInt(Object, long, int)
 */
 public native int arrayBaseOffset(Class<?> arrayClass);
 
 
 /**
 * Report the size in bytes of a native pointer, as stored via {@link
 * #putAddress}. This value will be either 4 or 8. Note that the sizes of
 * other primitive types (as stored in native memory blocks) is determined
 * fully by their information content.
 */
 public native int addressSize();
 
 /** The value of {@code addressSize()} */
 public static final int ADDRESS_SIZE = theUnsafe.addressSize();
 
 /**
 * Report the size in bytes of a native memory page (whatever that is).
 * This value will always be a power of two.
 */
 public native int pageSize();
 
 
 /// random trusted operations from JNI:
 
 /**
 * Tell the VM to define a class, without security checks. By default, the
 * class loader and protection domain come from the caller's class.
 */
 public native Class<?> defineClass(String name, byte[] b, int off, int len,
 ClassLoader loader,
 ProtectionDomain protectionDomain);
 
 /**
 * Define a class but do not make it known to the class loader or system dictionary.
 * 
 * For each CP entry, the corresponding CP patch must either be null or have
 * the a format that matches its tag:
 * <ul>
 * <li>Integer, Long, Float, Double: the corresponding wrapper object type from java.lang
 * <li>Utf8: a string (must have suitable syntax if used as signature or name)
 * <li>Class: any java.lang.Class object
 * <li>String: any object (not just a java.lang.String)
 * <li>InterfaceMethodRef: (NYI) a method handle to invoke on that call site's arguments
 * </ul>
 * @params hostClass context for linkage, access control, protection domain, and class loader
 * @params data bytes of a class file
 * @params cpPatches where non-null entries exist, they replace corresponding CP entries in data
 */
 public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches);
 
 
 /** Allocate an instance but do not run any constructor.
 Initializes the class if it has not yet been. */
 public native Object allocateInstance(Class<?> cls)
 throws InstantiationException;
 
 /** Lock the object. It must get unlocked via {@link #monitorExit}. */
 public native void monitorEnter(Object o);
 
 /**
 * Unlock the object. It must have been locked via {@link
 * #monitorEnter}.
 */
 public native void monitorExit(Object o);
 
 /**
 * Tries to lock the object. Returns true or false to indicate
 * whether the lock succeeded. If it did, the object must be
 * unlocked via {@link #monitorExit}.
 */
 public native boolean tryMonitorEnter(Object o);
 
 /** Throw the exception without telling the verifier. */
 public native void throwException(Throwable ee);
 
 
 /**
 * Atomically update Java variable to <tt>x</tt> if it is currently
 * holding <tt>expected</tt>.
 * @return <tt>true</tt> if successful
 */
 public final native boolean compareAndSwapObject(Object o, long offset,
 Object expected,
 Object x);
 
 /**
 * Atomically update Java variable to <tt>x</tt> if it is currently
 * holding <tt>expected</tt>.
 * @return <tt>true</tt> if successful
 */
 public final native boolean compareAndSwapInt(Object o, long offset,
 int expected,
 int x);
 
 /**
 * Atomically update Java variable to <tt>x</tt> if it is currently
 * holding <tt>expected</tt>.
 * @return <tt>true</tt> if successful
 */
 public final native boolean compareAndSwapLong(Object o, long offset,
 long expected,
 long x);
 
 /**
 * Fetches a reference value from a given Java variable, with volatile
 * load semantics. Otherwise identical to {@link #getObject(Object, long)}
 */
 public native Object getObjectVolatile(Object o, long offset);
 
 /**
 * Stores a reference value into a given Java variable, with
 * volatile store semantics. Otherwise identical to {@link #putObject(Object, long, Object)}
 */
 public native void putObjectVolatile(Object o, long offset, Object x);
 
 
 /**
 * Version of {@link #putObjectVolatile(Object, long, Object)}
 * that does not guarantee immediate visibility of the store to
 * other threads. This method is generally only useful if the
 * underlying field is a Java volatile (or if an array cell, one
 * that is otherwise only accessed using volatile accesses).
 */
 public native void putOrderedObject(Object o, long offset, Object x);
 
 
 /**
 * Unblock the given thread blocked on <tt>park</tt>, or, if it is
 * not blocked, cause the subsequent call to <tt>park</tt> not to
 * block. Note: this operation is "unsafe" solely because the
 * caller must somehow ensure that the thread has not been
 * destroyed. Nothing special is usually required to ensure this
 * when called from Java (in which there will ordinarily be a live
 * reference to the thread) but this is not nearly-automatically
 * so when calling from native code.
 * @param thread the thread to unpark.
 *
 */
 public native void unpark(Object thread);
 
 /**
 * Block current thread, returning when a balancing
 * <tt>unpark</tt> occurs, or a balancing <tt>unpark</tt> has
 * already occurred, or the thread is interrupted, or, if not
 * absolute and time is not zero, the given time nanoseconds have
 * elapsed, or if absolute, the given deadline in milliseconds
 * since Epoch has passed, or spuriously (i.e., returning for no
 * "reason"). Note: This operation is in the Unsafe class only
 * because <tt>unpark</tt> is, so it would be strange to place it
 * elsewhere.
 */
 public native void park(boolean isAbsolute, long time);
 
 /**
 * Gets the load average in the system run queue assigned
 * to the available processors averaged over various periods of time.
 * This method retrieves the given <tt>nelem</tt> samples and
 * assigns to the elements of the given <tt>loadavg</tt> array.
 * The system imposes a maximum of 3 samples, representing
 * averages over the last 1, 5, and 15 minutes, respectively.
 *
 * @params loadavg an array of double of size nelems
 * @params nelems the number of samples to be retrieved and
 * must be 1 to 3.
 *
 * @return the number of samples actually retrieved; or -1
 * if the load average is unobtainable.
 */
 public native int getLoadAverage(double[] loadavg, int nelems);
 
 // The following contain CAS-based Java implementations used on
 // platforms not supporting native instructions
 
 /**
 * Atomically adds the given value to the current value of a field
 * or array element within the given object <code>o</code>
 * at the given <code>offset</code>.
 *
 * @param o object/array to update the field/element in
 * @param offset field/element offset
 * @param delta the value to add
 * @return the previous value
 * @since 1.8
 */
 public final int getAndAddInt(Object o, long offset, int delta) {
 int v;
 do {
 v = getIntVolatile(o, offset);
 } while (!compareAndSwapInt(o, offset, v, v + delta));
 return v;
 } 
 
 /**
 * Atomically exchanges the given reference value with the current
 * reference value of a field or array element within the given
 * object <code>o</code> at the given <code>offset</code>.
 *
 * @param o object/array to update the field/element in
 * @param offset field/element offset
 * @param newValue new value
 * @return the previous value
 * @since 1.8
 */
 public final Object getAndSetObject(Object o, long offset, Object newValue) {
 Object v;
 do {
 v = getObjectVolatile(o, offset);
 } while (!compareAndSwapObject(o, offset, v, newValue));
 return v;
 }
 
 
 /**
 * Ensures lack of reordering of loads before the fence
 * with loads or stores after the fence.
 * @since 1.8
 */
 public native void loadFence();
 
 /**
 * Ensures lack of reordering of stores before the fence
 * with loads or stores after the fence.
 * @since 1.8
 */
 public native void storeFence();
 
 /**
 * Ensures lack of reordering of loads or stores before the fence
 * with loads or stores after the fence.
 * @since 1.8
 */
 public native void fullFence();
 
 /**
 * Throws IllegalAccessError; for use by the VM.
 * @since 1.8
 */
 private static void throwIllegalAccessError() {
 throw new IllegalAccessError();
 }
 
 }