关于 sun.misc.Unsafe

今天在看java.util.concurrent.atomic包的源代码的时候看到原来这个包的底层都调用了sun.misc.Unsafe这个类，这个类其实是个native的实现类，他的神奇之处就在于它可以直接访问内存地址。如果你能直接调用它，那么就意味着你可以绕开一切lock而直接操作地址，虽然危险却有相当的性能吸引力。（ 源代码 ）  它十分高效，因为它的实现基于CAS （Compare And Swap）。之所以名字叫不安全，是因为它可以直接访问内存，必须受信任的类才能使用它。如果你直接在eclipse里面引入他，有可能会报错。你需要修改eclipse里面的 preference-> java compler-> error/warning -> deprecated/districted API. Error -> Warm
它是单例的：

private static final Unsafe theUnsafe = new Unsafe();

 

1. 通过反射得到Unsafe实例

package jdk.unsafe;

import java.lang.reflect.Field;
import sun.misc.Unsafe;

@SuppressWarnings("restriction")
public class UnsafeClass {
	private static Unsafe unsafe;
	static {
		try {
			Field field = Unsafe.class.getDeclaredField("theUnsafe");
			field.setAccessible(true);
			unsafe = (Unsafe) field.get(null);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	public static Unsafe getInstance() {
		return unsafe;
	}
}

 

 

2. 给出待分析或修改的类

 

package jdk.unsafe;

@SuppressWarnings("unused")
public class CustomClass {

	private byte byteField = 1;

	private short shorteField = 2;

	private int intField = 3;

	private long longField = 6;

	private double doubleField = 5.5;

	private char charField = 'a';

	private String strField = "abc";

}

 

 

3. 测试类

 

package jdk.unsafe;

import java.lang.reflect.Field;

import sun.misc.Unsafe;

@SuppressWarnings("restriction")
public class UnsafeTest {
	private static final Unsafe unsafe = UnsafeClass.getInstance();
	private static final Class<CustomClass> customClass = CustomClass.class;
	private static final Field[] fields = customClass.getDeclaredFields();

	public static void main(String[] args) throws Exception {
		printFieldOffset(fields);
		CustomClass obj = new CustomClass();
		long longFieldOffset = unsafe.objectFieldOffset(
                            customClass.getDeclaredField("longField"));
		System.out.println("longFieldOffset:" + longFieldOffset);
		System.out.println("longField by unsafe: " + unsafe.getLong(obj, longFieldOffset));

		/**
		 * public final native boolean compareAndSwapLong(Object o, long
		 * offset,long expected, long newValue);
		 */
		System.out.println("change result: " + unsafe.compareAndSwapLong(obj, 
				longFieldOffset, 0, 100));
		System.out.println("longField by unsafe after update failed: " 
				+ unsafe.getLong(obj, longFieldOffset));

		System.out.println("change result: " + unsafe.compareAndSwapLong(obj, 
                                longFieldOffset, 6, 100));
		System.out.println("longField by unsafe after updatge successfully: " 
				+ unsafe.getLong(obj, longFieldOffset));

		unsafe.putLong(obj, longFieldOffset, 200);
		System.out.println("longField by unsafe after put a new value: " + 
				unsafe.getLong(obj, longFieldOffset));

	}

	private static void printFieldOffset(Field[] fields) {
		System.out.println("----------------------Field Offset Begin---------------------");
		for (Field f : fields) {
			System.out.println(f.getName() + ":" + unsafe.objectFieldOffset(f));
		}
		System.out.println("----------------------Field Offset End------------------------");
	}

}
 

 

4. 输出结果

 

----------------------Field Offset Begin---------------------
byteField:32
shorteField:28
intField:24
longField:8
doubleField:16
charField:30
strField:36
----------------------Field Offset End------------------------
longFieldOffset:8
longField by unsafe: 6
change result: false
longField by unsafe after update failed: 6
change result: true
longField by unsafe after updatge successfully: 100
longField by unsafe after put a new value: 200

 

5. 结果分析

a）通过unsafe可以直接访问和修改对象的属性，即使它是私有属性

b）没有lock和wait