/*
* %W% %E%
*
* Copyright (c) 2006,2010 Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package jef.tools;
//package sun.misc;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.security.ProtectionDomain;
/**
* 该类是JDK中用于底层操作的对外接口，功能极其强大。
* 将这个类放在这里不是为了测试，而是为了查看其API（这个类的API在JDK-DOC里是没有的），并将其翻译为中文。
* 除非你明白你在做什么，否则不要使用这个类相关的方法。
*
* 关于此类的JDK兼容性问题，目前检查发现SUN/IBM等实现均已包含此类
*
* 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.
* 这个类提供了一个更底层的操作并且应该在受信任的代码中使用。可以通过内存地址
* 存取fields,如果给出的内存地址是无效的那么会有一个不确定的运行表现。
*
*
*
* ---------------------------
* 关于对象字段访问的几种方法的差异，写字段有以下方式
* 1、直接put,如 putInt, putLong, putObject 9个方法
* 2、Volatile设置，9个方法。put__Volatile。java默认对象模型在设置值的时候是无序的，不能保证其值在其他线程的可见性，使用此方法赋值相当于该字段是volatile的。能保证多线程下该次设置的可见性。
* （速度稍慢）
* 3、putOrderedObject/putOrderedInt/putOrderedLong，3个方法。其效果等同于put__Volatile的延迟版本，其效果是不能保证其他线程的可见性的的。但是保证有序设置。之所以三个方法就够了，是因为
* java内部实现中，除了long和double以外，其他对象内部实现方式都是以int来进行的，即long和double可以互相表示，int和其他5种基本类型可以互相表示。
* 4、compareAndSwapXXX，即传说中的CAS操作，即比较并赋值，这一模型直接通过CPU原语支持，能保证比较和赋值的原子性。能有效提高多线程下并行操作的效率。
*
* 相应的读取字段就只有两种方式，即getXXX 和 getXxxVolatile两个，功能顾名思义。
* --------------------------
* Java轻量级锁
* monitorEnter、monitorExit/ tryMonitorEnter用于JDK6引入的轻量级锁的加锁和解锁，
* 具体原理较为复杂，参见 http://blog.csdn.net/songylwq/article/details/5585734
*
* @author John R. Rose
* @translator mr.jiyi@gmail.com
*
* @version %I%, %E%
* @see #getUnsafe
*/
public final class Unsafe {
private static native void registerNatives();
static {
registerNatives();
}
private Unsafe() {}
private static final Unsafe theUnsafe = new Unsafe();
/**
* Provides the caller with the capability of performing unsafe
* operations.
*
* <p> 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.
*
* <p> 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.
*
* <p> 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.
*/
public static Unsafe getUnsafe() {
Class cc = sun.reflect.Reflection.getCallerClass(2);
if (cc.getClassLoader() != null)
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.
* <p>
* 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><em>th</em> element of the array.
*
* </ul>
* <p>
* 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.
* <p>
* This method refers to a variable by means of two parameters, and so
* it provides (in effect) a <em>double-register</em> 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
* <em>single-register</em> 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.
* <p>
* 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.
* <p>
* 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.
* <p>
* 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);
/** @see #getInt(Object, long) */
public native boolean getBoolean(Object o, long offset);
/** @see #putInt(Object, int, int) */
public native void putBoolean(Object o, long offset, boolean x);
/** @see #getInt(Object, long) */
public native byte getByte(Object o, long offset);
/** @see #putInt(Object, int, int) */
public native void putByte(Object o, long offset, byte x);
/** @see #getInt(Object, long) */
public native short getShort(Object o, long offset);
/** @see #putInt(Object, int, int) */
public native void putShort(Object o, long offset, short x);
/** @see #getInt(Object, long) */
public native char getChar(Object o, long offset);
/** @see #putInt(Object, int, int) */
public native void putChar(Object o, long offset, char x);
/** @see #getInt(Object, long) */
public native long getLong(Object o, long offset);
/** @see #putInt(Object, int, int) */
public native void putLong(Object o, long offset, long x);
/** @see #getInt(Object, long) */
public native float getFloat(Object o, long offset);
/** @see #putInt(Object, int, int) */
public native void putFloat(Object o, long offset, float x);
/** @see #getInt(Object, long) */
public native double getDouble(Object o, long offset);
/** @see #putInt(Object, int, int) */
public native void putDouble(Object o, long offset, double 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);
/** @see #getByte(long) */
public native short getShort(long address);
/** @see #putByte(long, byte) */
public native void putShort(long address, short x);
/** @see #getByte(long) */
public native char getChar(long address);
/** @see #putByte(long, byte) */
public native void putChar(long address, char x);
/** @see #getByte(long) */
public native int getInt(long address);
/** @see #putByte(long, byte) */
public native void putInt(long address, int x);
/** @see #getByte(long) */
public native long getLong(long address);
/** @see #putByte(long, byte) */
public native void putLong(long address, long x);
/** @see #getByte(long) */
public native float getFloat(long address);
/** @see #putByte(long, byte) */
public native void putFloat(long address, float x);
/** @see #getByte(long) */
public native double getDouble(long address);
/** @see #putByte(long, byte) */
public native void putDouble(long address, double 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.
*
* <p> 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.
*
* <p> 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).
*/
public native void setMemory(long address, long bytes, byte value);
/**
* Sets all bytes in a given block of memory to a copy of another
* block.
*
* <p>This method determines each block's base address by means of two parameters,
* and so it provides (in effect) a <em>double-register</em> addressing mode,
* as discussed in {@link #getInt(Object,long)}. When the object reference is null,
* the offset supplies an absolute base address.
*
* <p>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'.
*
* 将对象的整段内存拷贝到另外一个对象处，比如数组拷贝
* @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 <em>single-register</em> 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);
/// random queries
/**
* This constant differs from all results that will ever be returned from
* {@link #staticFieldOffset}, {@link #objectFieldOffset},
* or {@link #arrayBaseOffset}.
*/
public static final int INVALID_FIELD_OFFSET = -1;
/**
* Returns the offset of a field, truncated to 32 bits.
* This method is implemented as follows:
* <blockquote><pre>
* public int fieldOffset(Field f) {
* if (Modifier.isStatic(f.getModifiers()))
* return (int) staticFieldOffset(f);
* else
* return (int) objectFieldOffset(f);
* }
* </pre></blockquote>
* @deprecated As of 1.4.1, use {@link #staticFieldOffset} for static
* fields and {@link #objectFieldOffset} for non-static fields.
*/
@Deprecated
public int fieldOffset(Field f) {
if (Modifier.isStatic(f.getModifiers()))
return (int) staticFieldOffset(f);
else
return (int) objectFieldOffset(f);
}
/**
* Returns the base address for accessing some static field
* in the given class. This method is implemented as follows:
* <blockquote><pre>
* public Object staticFieldBase(Class c) {
* Field[] fields = c.getDeclaredFields();
* for (int i = 0; i < fields.length; i++) {
* if (Modifier.isStatic(fields[i].getModifiers())) {
* return staticFieldBase(fields[i]);
* }
* }
* return null;
* }
* </pre></blockquote>
* @deprecated As of 1.4.1, use {@link #staticFieldBase(Field)}
* to obtain the base pertaining to a specific {@link Field}.
* This method works only for JVMs which store all statics
* for a given class in one place.
*/
@Deprecated
public Object staticFieldBase(Class c) {
Field[] fields = c.getDeclaredFields();
for (int i = 0; i < fields.length; i++) {
if (Modifier.isStatic(fields[i].getModifiers())) {
return staticFieldBase(fields[i]);
}
}
return null;
}
/**
* 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.
*
* <p>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.
*
* <p>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}.
* <p>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.
*
* <p>Any given field will always have the same offset, and no two distinct
* fields of the same class will ever have the same offset.
*
* <p>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)
*
* 返回指定静态field的内存地址偏移量,在这个类的其他方法中这个值只是被用作一个访问
* 特定field的一个方式。这个值对于 给定的field是唯一的，并且后续对该方法的调用都应该
* 返回相同的值。
*
* @param field the field whose offset should be returned.
* 需要返回偏移量的field
* @return the offset of the given field.
* 指定field的偏移量
*/
public native long objectFieldOffset(Field f);
/**
* Report the location of a given static field, in conjunction with {@link
* #staticFieldOffset}.
* <p>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);
/**
* 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.
*
* 获取给定数组中第一个元素的偏移地址。
* 为了存取数组中的元素，这个偏移地址与<a href="#arrayIndexScale"><code>arrayIndexScale
* </code></a>方法的非0返回值一起被使用。
* @param arrayClass the class for which the first element's address should
* be obtained.
* 第一个元素地址被获取的class
* @return the offset of the first element of the array class.
* 数组第一个元素 的偏移地址
* @see arrayIndexScale(Class)
* @see #getInt(Object, long)
* @see #putInt(Object, long, int)
*/
public native int arrayBaseOffset(Class arrayClass);
/**
* Report the scale factor for addressing elements in the storage
* allocation of a given array class. However, arrays of "narrow" types
* will generally not work properly with accessors like {@link
* #getByte(Object, int)}, so the scale factor for such classes is reported
* as zero.
* 获取用户给定数组寻址的换算因子.一个合适的换算因子不能返回的时候(例如：基本类型),
* 即获取数组每个元素所占的字节数 （偏移量）
* 这个返回值能够与<a href="#arrayBaseOffset"><code>arrayBaseOffset</code>
* </a>一起使用去存取这个数组class中的元素
*
* @see #arrayBaseOffset
* @see #getInt(Object, long)
* @see #putInt(Object, long, int)
*/
public native int arrayIndexScale(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();
/**
* 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);
public native Class defineClass(String name, byte[] b, int off, int len);
/** Allocate an instance but do not run any constructor.
Initializes the class if it has not yet been.
* 创建对象，不会运行对象的初始化方法<init>
*/
public native Object allocateInstance(Class cls)
throws InstantiationException;
/** Lock the object. It must get unlocked via {@link #monitorExit}.
*
* monitorEnter、monitorExit/ tryMonitorEnter用于JDK6引入的轻量级锁的加锁和解锁，
* 具体原理较为复杂，参见 http://blog.csdn.net/songylwq/article/details/5585734
*/
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
*
* 在obj的offset位置比较field和期望的值，如果相同则更新。这个方法
* 的操作应该是原子的，因此提供了一种不可中断的方式更新integer field。
*
* @param obj the object containing the field to modify.
* 包含要修改field的对象
* @param offset the offset of the integer field within <code>obj</code>.
* <code>obj</code>中整型field的偏移量
* @param expect the expected value of the field.
* 希望field中存在的值
* @param update the new value of the field if it equals <code>expect</code>.
* 如果期望值expect与field的当前值相同，设置filed的值为这个新值
* @return true if the field was changed.
* 如果field的值被更改
*/
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
*
* 在obj的offset位置比较 field和期望的值，如果相同则更新。这个方法
* 的操作应该是原子的，因此提供了一种不可中断的方式更新field。
*
* 注意，其他所有int boolean byte short float char都可以用这个方法
*
* @param obj the object containing the field to modify.
* 包含要修改field的对象
* @param offset the offset of the integer field within <code>obj</code>.
* <code>obj</code>中整型field的偏移量
* @param expect the expected value of the field.
* 希望field中存在的值
* @param update the new value of the field if it equals <code>expect</code>.
* 如果期望值expect与field的当前值相同，设置filed的值为这个新值
* @return true if the field was changed.
* 如果field的值被更改
*/
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
* 在obj的offset位置比较field和期望的值，如果相同则更新。这个方法
* 的操作应该是原子的，因此提供了一种不可中断的方式更新integer field。
* 注意，其他所有long double都可以用这个方法
*
* @param obj the object containing the field to modify.
* 包含要修改field的对象
* @param offset the offset of the integer field within <code>obj</code>.
* <code>obj</code>中整型field的偏移量
* @param expect the expected value of the field.
* 希望field中存在的值
* @param update the new value of the field if it equals <code>expect</code>.
* 如果期望值expect与field的当前值相同，设置filed的值为这个新值
* @return true if the field was changed.
* 如果field的值被更改
*/
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);
/** Volatile version of {@link #getInt(Object, long)} */
public native int getIntVolatile(Object o, long offset);
/** Volatile version of {@link #putInt(Object, long, int)}
* 设置obj对象中offset偏移地址对应的整型field的值为指定值。支持volatile store语义
* 在目前大多数的处理器架构上，volatile 读操作开销非常低 —— 几乎和非 volatile 读操作一样。
* 而 volatile 写操作的开销要比非 volatile 写操作多很多，因为要保证可见性需要实现内存界定（Memory Fence），即便如此，volatile 的总开销仍然要比锁获取低。
*
* @param obj the object containing the field to modify.
* 包含需要修改field的对象
* @param offset the offset of the integer field within <code>obj</code>.
* <code>obj</code>中整型field的偏移量
* @param value the new value of the field.
* field将被设置的新值
* */
public native void putIntVolatile(Object o, long offset, int x);
/** Volatile version of {@link #getBoolean(Object, long)} */
public native boolean getBooleanVolatile(Object o, long offset);
/** Volatile version of {@link #putBoolean(Object, long, boolean)} */
public native void putBooleanVolatile(Object o, long offset, boolean x);
/** Volatile version of {@link #getByte(Object, long)} */
public native byte getByteVolatile(Object o, long offset);
/** Volatile version of {@link #putByte(Object, long, byte)} */
public native void putByteVolatile(Object o, long offset, byte x);
/** Volatile version of {@link #getShort(Object, long)} */
public native short getShortVolatile(Object o, long offset);
/** Volatile version of {@link #putShort(Object, long, short)} */
public native void putShortVolatile(Object o, long offset, short x);
/** Volatile version of {@link #getChar(Object, long)} */
public native char getCharVolatile(Object o, long offset);
/** Volatile version of {@link #putChar(Object, long, char)} */
public native void putCharVolatile(Object o, long offset, char x);
/** Volatile version of {@link #getLong(Object, long)} */
public native long getLongVolatile(Object o, long offset);
/** Volatile version of {@link #putLong(Object, long, long)} */
public native void putLongVolatile(Object o, long offset, long x);
/** Volatile version of {@link #getFloat(Object, long)} */
public native float getFloatVolatile(Object o, long offset);
/** Volatile version of {@link #putFloat(Object, long, float)} */
public native void putFloatVolatile(Object o, long offset, float x);
/** Volatile version of {@link #getDouble(Object, long)} */
public native double getDoubleVolatile(Object o, long offset);
/** Volatile version of {@link #putDouble(Object, long, double)} */
public native void putDoubleVolatile(Object o, long offset, double 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).
*
* 设置obj对象中offset偏移地址对应的整型field的值为指定值。这是一个有序或者
* 有延迟的<code>putIntVolatile</cdoe>方法，并且不保证值的改变被其他线程立
* 即看到。只有在field被<code>volatile</code>修饰并且期望被意外修改的时候
* 使用才有用。
*
* @param obj the object containing the field to modify.
* 包含需要修改field的对象
* @param offset the offset of the integer field within <code>obj</code>.
* <code>obj</code>中整型field的偏移量
* @param value the new value of the field.
* field将被设置的新值
*/
public native void putOrderedObject(Object o, long offset, Object x);
/** Ordered/Lazy version of {@link #putIntVolatile(Object, long, int)} */
public native void putOrderedInt(Object o, long offset, int x);
/** Ordered/Lazy version of {@link #putLongVolatile(Object, long, long)} */
public native void putOrderedLong(Object o, long offset, long 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.
* 释放被<a href="#park"><code>park</code></a>创建的在一个线程上的阻塞.这个
* 方法也可以被使用来终止一个先前调用<code>park</code>导致的阻塞.
* 这个操作操作时不安全的,因此线程必须保证是活的.这是java代码不是native代码。
* @param thread the thread to unblock.
* 要解除阻塞的线程
*
*/
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.
* 阻塞一个线程直到<a href="#unpark"><code>unpark</code></a>出现、线程
* 被中断或者timeout时间到期。如果一个<code>unpark</code>调用已经出现了，
* 这里只计数。timeout为0表示永不过期.当<code>isAbsolute</code>为true时，
* timeout是相对于新纪元之后的毫秒。否则这个值就是超时前的纳秒数。这个方法执行时
* 也可能不合理地返回(没有具体原因)
*
* @param isAbsolute true if the timeout is specified in milliseconds from
* the epoch.
* 如果为true timeout的值是一个相对于新纪元之后的毫秒数
* @param time either the number of nanoseconds to wait, or a time in
* milliseconds from the epoch to wait for.
* 可以是一个要等待的纳秒数，或者是一个相对于新纪元之后的毫秒数直到
* 到达这个时间点
*/
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);
}