JDK中Unsafe类详解
http://hg.openjdk.java.net/jdk7/jdk7/jdk/file/9b8c96f96a0f/src/share/classes/sun/misc/Unsafe.java
http://hg.openjdk.java.net/jdk7/jdk7/hotspot/file/tip/src/share/vm/prims/unsafe.cpp
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/sun/misc/Unsafe.java
http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/share/vm/prims/unsafe.cpp
案例代码
package com.dsp.unsafe;
import java.io.Serializable;
import java.lang.reflect.Field;
import java.util.Arrays;
import java.util.concurrent.locks.ReentrantLock;
import com.alibaba.fastjson.JSON;
import com.dsp.json.Person;
import sun.misc.Unsafe;
@SuppressWarnings("restriction")
public class UnsafeDemo {
static class Test {
private final int x;
Test(int x) {
this.x = x;
System.out.println("Test ctor");
}
int getX() {
return x;
}
}
public static void main(String[] args) throws InstantiationException, NoSuchFieldException {
// 获得一个UnSafe实例
Unsafe unsafe = null;
try {
Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
unsafe = (Unsafe) f.get(null);
} catch (NoSuchFieldException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
if (unsafe != null) {
try {
// 构造一个对象，且不调用其构造函数
Test test = (Test) unsafe.allocateInstance(Test.class);
// 得到一个对象内部属性的地址
long x_addr = unsafe.objectFieldOffset(Test.class.getDeclaredField("x"));
// 直接给此属性赋值
unsafe.getAndSetInt(test, x_addr, 47);
System.out.println(test.getX());
} catch (InstantiationException e) {
e.printStackTrace();
} catch (NoSuchFieldException e) {
e.printStackTrace();
}
}
// 通过地址操作数组
if (unsafe != null) {
final int INT_BYTES = 4;
int[] data = new int[10];
System.out.println(Arrays.toString(data));
long arrayBaseOffset = unsafe.arrayBaseOffset(int[].class);
System.out.println("Array address is :" + arrayBaseOffset);
long arrayBaseOffset2 = unsafe.arrayBaseOffset(double[].class);
System.out.println("Array address is :" + arrayBaseOffset2);
unsafe.putInt(data, arrayBaseOffset, 47);
unsafe.putInt(data, arrayBaseOffset + INT_BYTES * 8, 43);
System.out.println(Arrays.toString(data));
}
// CAS
if (unsafe != null) {
Test test = (Test) unsafe.allocateInstance(Test.class);
long x_addr = unsafe.objectFieldOffset(Test.class.getDeclaredField("x"));
unsafe.getAndSetInt(test, x_addr, 47);
unsafe.compareAndSwapInt(test, x_addr, 47, 78);
System.out.println("After CAS:" + test.getX());
}
}
@SuppressWarnings("deprecation")
public static void mainB(String[] args)
throws SecurityException, NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafe.setAccessible(true);
Unsafe UNSAFE = (Unsafe) theUnsafe.get(null);
System.out.println(UNSAFE);
byte[] data = new byte[10];
System.out.println(Arrays.toString(data));
int byteArrayBaseOffset = UNSAFE.arrayBaseOffset(byte[].class);
System.out.println(byteArrayBaseOffset);
UNSAFE.putByte(data, byteArrayBaseOffset, (byte) 1);
UNSAFE.putByte(data, byteArrayBaseOffset + 5, (byte) 5);
System.out.println(Arrays.toString(data));
UNSAFE.setMemory(data, byteArrayBaseOffset, 1, (byte) 2);
UNSAFE.setMemory(data, byteArrayBaseOffset + 5, 1, (byte) 6);
System.out.println(Arrays.toString(data));
}
@SuppressWarnings({ "unused", "rawtypes" })
public static void mainA(String[] args)
throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException {
Field field = Unsafe.class.getDeclaredField("theUnsafe");
field.setAccessible(true);
Unsafe unsafe = (Unsafe) field.get(null);
long allocateMemory = unsafe.allocateMemory(1024);
long theUnsafeOffset = unsafe.staticFieldOffset(field);
System.out.println(theUnsafeOffset);
/********************************************************************************
* 获取对象中某字段在内存中的偏移量
*/
// 开始使用unsafe对象，分别找到Person对象中name属性和age属性的内存地址偏移量
// 首先是Person类中的name属性，在内存中设定的偏移位置
Field field2 = Person.class.getDeclaredField("name");
// 一旦这个类实例化后，该属性在内存中的偏移位置
long offset2 = unsafe.objectFieldOffset(field2);
System.out.println("name offset = " + offset2);
/*
* 然后是Person类中的age属性，在内存中设定的偏移位置
*/
Field age3 = Person.class.getDeclaredField("age");
long ageOffset3 = unsafe.objectFieldOffset(age3);
System.out.println("age offset = " + ageOffset3);
/********************************************************************************
* 修改某个字段的数据
*/
/*
* 修改字段数据
*/
Person person = new Person();
person.setName("dsp");
person.setAge(20);
/*
* 获取age属性的内存地址偏移量
*/
Field ageField = Person.class.getDeclaredField("age");
long ageOffset = unsafe.objectFieldOffset(ageField);
/*
* 比较并修改值 1、需要修改的对象 2、更改属性的内存偏移量 3、预期的值 4、设置的新值
*/
if (unsafe.compareAndSwapInt(person, ageOffset, 20, 26)) {
System.out.println("修改数据成功");
} else {
System.out.println("修改数据失败");
}
System.out.println(JSON.toJSONString(person));
int ss, ts;
try {
Class<Segment[]> sc = Segment[].class;
SBASE = unsafe.arrayBaseOffset(sc);
ss = unsafe.arrayIndexScale(sc);
} catch (Exception e) {
throw new Error(e);
}
SSHIFT = 31 - Integer.numberOfLeadingZeros(ss);
System.out.println("SBASE=" + SBASE);
System.out.println("ss=" + ss);
System.out.println("SSHIFT=" + SSHIFT);
int ARRAY_INT_BASE_OFFSET = unsafe.arrayBaseOffset(int[].class);
int ARRAY_INT_INDEX_SCALE = unsafe.arrayIndexScale(int[].class);
System.out.println("ARRAY_INT_BASE_OFFSET=" + ARRAY_INT_BASE_OFFSET);
System.out.println("ARRAY_INT_INDEX_SCALE=" + ARRAY_INT_INDEX_SCALE);
}
// Unsafe mechanics
private static long SBASE;
private static long SSHIFT;
static final class Segment<K, V> extends ReentrantLock implements Serializable {
/*
* Segments maintain a table of entry lists that are always kept in a consistent
* state, so can be read (via volatile reads of segments and tables) without
* locking. This requires replicating nodes when necessary during table
* resizing, so the old lists can be traversed by readers still using old
* version of table.
*
* This class defines only mutative methods requiring locking. Except as noted,
* the methods of this class perform the per-segment versions of
* ConcurrentHashMap methods. (Other methods are integrated directly into
* ConcurrentHashMap methods.) These mutative methods use a form of controlled
* spinning on contention via methods scanAndLock and scanAndLockForPut. These
* intersperse tryLocks with traversals to locate nodes. The main benefit is to
* absorb cache misses (which are very common for hash tables) while obtaining
* locks so that traversal is faster once acquired. We do not actually use the
* found nodes since they must be re-acquired under lock anyway to ensure
* sequential consistency of updates (and in any case may be undetectably
* stale), but they will normally be much faster to re-locate. Also,
* scanAndLockForPut speculatively creates a fresh node to use in put if no node
* is found.
*/
private static final long serialVersionUID = 2249069246763182397L;
/**
* The maximum number of times to tryLock in a prescan before possibly blocking
* on acquire in preparation for a locked segment operation. On multiprocessors,
* using a bounded number of retries maintains cache acquired while locating
* nodes.
*/
static final int MAX_SCAN_RETRIES = Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;
}
}
:）
