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1:概念
GcRoot是一个对象引用链的起点,引出它们指向的下一个节点,再以下个节点为起点,引出此节点指向的下一个结点。这样通过 GC Root
串成的一条线就叫引用链)直到所有的结点都遍历完毕,如果相关对象不在任意一个以 GC Root 为起点的引用链中,那么虚拟机就可以在内存不足的时候,回收 这个对象
2:GC Root对象有哪些
虚拟机栈 -----栈帧中的本地 变量表中引用的对象
本地方法栈 -----即一般说的 Native方法引用的对象
方法区中 类静态属性引用的对象
方法区中 常量引用的对象
3:分析各个GC Root对象实例
3.1 虚拟机栈中引用的对象
如下代码:a是栈帧中的本地变量,当 a= null 时,由于此时a充当了 GC Root对象,a 与原来指向的 实例 new Test()
断开了连接,所以对象会被回收。
publicclass Test { public static void main(String[] args) { Test a = new
Test(); a = null; } }
3.2 本地方法栈中JNI引用的对象
1: 所谓本地方法就是一个 java方法调用非 Java代码的接口,该方法不是有java实现的,可能有 C 或Python等其他语言实现的,java通过
jni 来调用本地方法
2:而本地方法是以库文件形式存放的,通过调用本地的库文件的内部方法,使java可以实现和本地机器的紧密联系。
3: 调用
java方法时,虚拟机会创建一个栈帧并压入java栈,而他调用本地方法时,虚拟机会保持java栈不变,不会爱java栈帧中压入新的栈,虚拟机只是简单地动态连接并直接调用指定的本地方法。
JNIEXPORT void JNICALL
Java_com_pecuyu_jnirefdemo_MainActivity_newStringNative(JNIEnv *env, jobject
instance,jstring jmsg) { ... // 缓存String的class jclass jc =
(*env)->FindClass(env, STRING_PATH); }
如上代码所示,当 java 调用以上本地方法时,jc 会被本地方法栈压入栈中, jc 就是我们说的本地方法栈中 JNI
的对象引用,因此只会在此本地方法执行完成后才会被释放。