JDK中Unsafe类详解

Java中Unsafe类详解

在openjdk8下看Unsafe源码

浅析Java中的原子操作

Java并发编程之LockSupport

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;
	}

}

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