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java的jni 问题大总结

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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>局部引用 vs. 全局引用</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 5 页（共13 页）</nobr></font></td>
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<p>当使用 JNI 编程时，会需要使用对 Java 对象的引用。缺省情况下，JNI 创建局部引用以确保它们可以被垃圾收集。由于这一点，您可能会因为尝试存储一个本地引用，以便稍后重用它而无意间编写出非法代码，如下面的代码样本所示：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
 1. /* This code is invalid! */
 2. static jmethodID mid;
 3. 
 4. JNIEXPORT jstring JNICALL
 5. Java_Sample1_accessMethod(JNIEnv *env, jobject obj)
 6. {
 7.    ...
 8.	 cls = (*env)-&gt;GetObjectClass(env, obj);
 9.    if (cls != 0)
10.       mid = (*env)-&gt;GetStaticMethodID(env, cls, "addInt", "(I)I");
11.    ...
12. }
</code>
</pre>
<p>因为第 10 行的错误，所以这个代码段是非法的。<code>mid</code> 是 <code>methodID</code>，并且 <code>GetStaticMethodID()</code> 返回 <code>methodID</code>。但是，返回的 <code>methodID</code> 是一个局部引用，而您不应该将一个局部引用赋给全局引用。而 <code>mid</code> 是一个全局引用。</p>
<p>在 <code>Java_Sample1_accessMethod()</code> 返回之后，<code>mid</code> 引用就不再有效，因为赋给它现在超出作用域以外的局部引用。尝试使用 <code>mid</code> 将导致错误结果或 JVM 崩溃。</p>
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