分类： Java
2017-02-07 16:49:15
说到堆外内存，那大家肯定想到堆内内存，这也是我们大家接触最多的，我们在jvm参数里通常设置-Xmx来指定我们的堆的最大值，不过这还不是我们理解的Java堆，-Xmx的值是新生代和老生代的和的最大值，我们在jvm参数里通常还会加一个参数-XX:MaxPermSize来指定持久代的最大值，那么我们认识的Java堆的最大值其实是-Xmx和-XX:MaxPermSize的总和，在分代算法下，新生代，老生代和持久代是连续的虚拟地址，因为它们是一起分配的，那么剩下的都可以认为是堆外内存(广义的)了，这些包括了jvm本身在运行过程中分配的内存，codecache，jni里分配的内存，DirectByteBuffer分配的内存等等
而作为java开发者，我们常说的堆外内存溢出了，其实是狭义的堆外内存，这个主要是指java.nio.DirectByteBuffer在创建的时候分配内存，我们这篇文章里也主要是讲狭义的堆外内存，因为它和我们平时碰到的问题比较密切
DirectByteBuffer通常用在通信过程中做缓冲池，在mina，netty等nio框架中屡见不鲜，先来看看JDK里的实现：
DirectByteBuffer(int cap) { // package-private super(-1, 0, cap, cap);
boolean pa = VM.isDirectMemoryPageAligned(); int ps = Bits.pageSize(); long size = Math.max(1L, (long)cap + (pa ? ps : 0));
Bits.reserveMemory(size, cap); long base = 0; try { base = unsafe.allocateMemory(size);
} catch (OutOfMemoryError x) {
Bits.unreserveMemory(size, cap); throw x;
} unsafe.setMemory(base, size, (byte) 0); if (pa && (base % ps != 0)) { // Round up to page boundary address = base + ps - (base & (ps - 1));
} else {
address = base;
}
cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
att = null;
}
通过上面的构造函数我们知道，真正的内存分配是使用的Bits.reserveMemory方法
static void reserveMemory(long size, int cap) { synchronized (Bits.class) { if (!memoryLimitSet && VM.isBooted()) {
maxMemory = VM.maxDirectMemory();
memoryLimitSet = true;
} // -XX:MaxDirectMemorySize limits the total capacity rather than the // actual memory usage, which will differ when buffers are page // aligned. if (cap <= maxMemory - totalCapacity) {
reservedMemory += size;
totalCapacity += cap; count++; return;
}
}
System.gc(); try {
Thread.sleep(100);
} catch (InterruptedException x) { // Restore interrupt status Thread.currentThread().interrupt();
} synchronized (Bits.class) { if (totalCapacity + cap > maxMemory) throw new OutOfMemoryError("Direct buffer memory");
reservedMemory += size;
totalCapacity += cap; count++;
}
}
通过上面的代码我们知道可以通过-XX:MaxDirectMemorySize来指定最大的堆外内存，那么我们首先引入两个问题
如果我们没有通过-XX:MaxDirectMemorySize来指定最大的堆外内存，那么默认的最大堆外内存是多少呢，我们还是通过代码来分析
上面的代码里我们看到调用了sun.misc.VM.maxDirectMemory()
private static long directMemory = 64 * 1024 * 1024;
// Returns the maximum amount of allocatable direct buffer memory.
// The directMemory variable is initialized during system initialization
// in the saveAndRemoveProperties method.
//
public static long maxDirectMemory() { return directMemory;
}
看到上面的代码之后是不是误以为默认的最大值是64M？其实不是的，说到这个值得从java.lang.System这个类的初始化说起
/**
* Initialize the system class. Called after thread initialization.
*/ private static void initializeSystemClass() { // VM might invoke JNU_NewStringPlatform() to set those encoding // sensitive properties (user.home, user.name, boot.class.path, etc.) // during "props" initialization, in which it may need access, via // System.getProperty(), to the related system encoding property that // have been initialized (put into "props") at early stage of the // initialization. So make sure the "props" is available at the // very beginning of the initialization and all system properties to // be put into it directly. props = new Properties();
initProperties(props); // initialized by the VM // There are certain system configurations that may be controlled by // VM options such as the maximum amount of direct memory and // Integer cache size used to support the object identity semantics // of autoboxing. Typically, the library will obtain these values // from the properties set by the VM. If the properties are for // internal implementation use only, these properties should be // removed from the system properties. // // See java.lang.Integer.IntegerCache and the // sun.misc.VM.saveAndRemoveProperties method for example. // // Save a private copy of the system properties object that // can only be accessed by the internal implementation. Remove // certain system properties that are not intended for public access. sun.misc.VM.saveAndRemoveProperties(props);
......
sun.misc.VM.booted();
}
上面这个方法在jvm启动的时候对System这个类做初始化的时候执行的，因此执行时间非常早，我们看到里面调用了sun.misc.VM.saveAndRemoveProperties(props):