單例模式在實際工作或面試中出現的頻率更高,double-checked lock是單例常見實作:
<b>public</b> <b>class</b> SimpleSingleton4 {
<b>private</b> <b>static</b> SimpleSingleton4 INSTANCE;
<b>private</b> SimpleSingleton4() {
}
<b>public</b> <b>static</b> SimpleSingleton4 getInstance() {
<b>if</b> (INSTANCE == <b>null</b>) {
<b>synchronized</b> (SimpleSingleton4.<b>class</b>) {
<b>if</b> (INSTANCE == <b>null</b>) {
INSTANCE = <b>new</b> SimpleSingleton4();
}
}
}
<b>return</b> INSTANCE;
}
}
這段代碼有問題:
<b>public</b> <b>static</b> SimpleSingleton4 getInstance() {
<b>if</b> (INSTANCE == <b>null</b>) {<font><i>//1</i></font><font>
<b>synchronized</b> (SimpleSingleton4.<b>class</b>) {</font><font><i>//2</i></font><font>
<b>if</b> (INSTANCE == <b>null</b>) {</font><font><i>//3</i></font><font>
INSTANCE = <b>new</b> SimpleSingleton4();</font><font><i>//4</i></font><font>
}
}
}
<b>return</b> INSTANCE;</font><font><i>//5</i></font><font>
}
</font>
你希望按1, 2, 3, 4, 金額 5順序執行。
但是Java虛拟機實際上會做一些優化,重新排列一些代碼指令。重排後的順序可能會變成: 1, 3, 2, 4, 5,這樣在多線程的情況下也會建立多個執行個體。重新排列的代碼可能如下所示:
<b>public</b> <b>static</b> SimpleSingleton4 getInstance() {
<b>if</b> (INSTANCE == <b>null</b>) {<font><i>//1</i></font><font>
<b>if</b> (INSTANCE == <b>null</b>) {</font><font><i>//3</i></font><font>
<b>synchronized</b> (SimpleSingleton4.<b>class</b>) {</font><font><i>//2</i></font><font>
INSTANCE = <b>new</b> SimpleSingleton4();</font><font><i>//4</i></font><font>
}
}
}
<b>return</b> INSTANCE;</font><font><i>//5</i></font><font>
}
</font>
有什麼解決辦法?
答:您可以将volatile關鍵字添加到 的定義中INSTANCE。具體代碼如下:
<b>private</b> <b>volatile</b> <b>static</b> SimpleSingleton7 INSTANCE;
<b>private</b> SimpleSingleton7() {
}
<b>public</b> <b>static</b> SimpleSingleton7 getInstance() {
<b>if</b> (INSTANCE == <b>null</b>) {
<b>synchronized</b> (SimpleSingleton7.<b>class</b>) {
<b>if</b> (INSTANCE == <b>null</b>) {
INSTANCE = <b>new</b> SimpleSingleton7();
}
}
}
<b>return</b> INSTANCE;
}