單例模式相信大家都知道,用過的人不在少數。之前寫過一篇博文《singleton模式四種線程安全的實現》,講訴了單例模式的四種寫法,並指出佔位符模式的寫法比較ok,詳見如下:
package com.effective.singleton;
public class Elvis
{
private static boolean flag = false;
private Elvis(){
}
private static class SingletonHolder{
private static final Elvis INSTANCE = new Elvis();
}
public static Elvis getInstance()
{
return SingletonHolder.INSTANCE;
}
public void doSomethingElse()
{
}
}
但這都是基於一個條件:確保不會通過反射機制調用私有的構造器。
這裏舉個例子,通過JAVA的反射機制來“攻擊”單例模式:
package com.effective.singleton;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
public class ElvisReflectAttack
{
public static void main(String[] args) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException
{
Class<?> classType = Elvis.class;
Constructor<?> c = classType.getDeclaredConstructor(null);
c.setAccessible(true);
Elvis e1 = (Elvis)c.newInstance();
Elvis e2 = Elvis.getInstance();
System.out.println(e1==e2);
}
}
運行結果:false
可以看到,通過反射獲取構造函數,然後調用setAccessible(true)就可以調用私有的構造函數,所有e1和e2是兩個不同的對象。
如果要抵禦這種攻擊,可以修改構造器,讓它在被要求創建第二個實例的時候拋出異常。
經修改後:
package com.effective.singleton;
public class ElvisModified
{
private static boolean flag = false;
private ElvisModified(){
synchronized(ElvisModified.class)
{
if(flag == false)
{
flag = !flag;
}
else
{
throw new RuntimeException("單例模式被侵犯!");
}
}
}
private static class SingletonHolder{
private static final ElvisModified INSTANCE = new ElvisModified();
}
public static ElvisModified getInstance()
{
return SingletonHolder.INSTANCE;
}
public void doSomethingElse()
{
}
}
測試代碼:
package com.effective.singleton;
import java.lang.reflect.Constructor;
public class ElvisModifiedReflectAttack
{
public static void main(String[] args)
{
try
{
Class<ElvisModified> classType = ElvisModified.class;
Constructor<ElvisModified> c = classType.getDeclaredConstructor(null);
c.setAccessible(true);
ElvisModified e1 = (ElvisModified)c.newInstance();
ElvisModified e2 = ElvisModified.getInstance();
System.out.println(e1==e2);
}
catch (Exception e)
{
e.printStackTrace();
}
}
}
運行結果:
Exception in thread "main" java.lang.ExceptionInInitializerError
at com.effective.singleton.ElvisModified.getInstance(ElvisModified.java:27)
at com.effective.singleton.ElvisModifiedReflectAttack.main(ElvisModifiedReflectAttack.java:17)
Caused by: java.lang.RuntimeException: 單例模式被侵犯!
at com.effective.singleton.ElvisModified.<init>(ElvisModified.java:16)
at com.effective.singleton.ElvisModified.<init>(ElvisModified.java:7)
at com.effective.singleton.ElvisModified$SingletonHolder.<clinit>(ElvisModified.java:22)
... 2 more
可以看到,成功的阻止了單例模式被破壞。
從JDK1.5開始,實現Singleton還有新的寫法,只需編寫一個包含單個元素的枚舉類型。推薦寫法:
package com.effective.singleton;
public enum SingletonClass
{
INSTANCE;
public void test()
{
System.out.println("The Test!");
}
}
測試代碼:
package com.effective;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import com.effective.singleton.SingletonClass;
public class TestMain
{
public static void main(String[] args) throws NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException
{
Class<SingletonClass> classType = SingletonClass.class;
Constructor<SingletonClass> c = (Constructor<SingletonClass>) classType.getDeclaredConstructor();
c.setAccessible(true);
c.newInstance();
}
}
運行結果:
Exception in thread "main" java.lang.NoSuchMethodException: com.effective.singleton.SingletonClass.<init>()
at java.lang.Class.getConstructor0(Unknown Source)
at java.lang.Class.getDeclaredConstructor(Unknown Source)
at com.effective.TestMain.main(TestMain.java:22)
由此可見這種寫法也可以防止單例模式被“攻擊”。
而且這種寫法也可以防止序列化破壞單例模式,具體不在舉例了。
單元素的枚舉類型已經成爲實現Singleton模式的最佳方法。