第 11 章 直接内存
1、直接内存概述
直接内存不属于虚拟机运行时数据区的一部分，也不是《java虚拟机规范》中定义的内存区域。它是java堆外的、直接向系统申请的内存区间。直接内存来源于nio，通过存在堆中的directbytebuffer操作native内存。通常，访问直接内存的速度会优于java堆。因此，出于性能考虑，读写频繁的场合可能会考虑使用直接内存。java的nio库允许java程序使用直接内存，用于数据缓冲区。
/**
* IO NIO (New IO / Non-Blocking IO)
* byte[] / char[] Buffer
* Stream Channel
*
* 查看直接内存的占用与释放
* @author shkstart shkstart@126.com
* @create 2020 0:22
*/
public class BufferTest {
private static final int BUFFER = 1024 * 1024 * 1024;//1GB
public static void main(String[] args){
//直接分配本地内存空间
ByteBuffer byteBuffer = ByteBuffer.allocateDirect(BUFFER);
System.out.println("直接内存分配完毕，请求指示！");
Scanner scanner = new Scanner(System.in);
scanner.next();
System.out.println("直接内存开始释放！");
byteBuffer = null;
System.gc();
scanner.next();
}
}呐，直接占用了 1G 的本地内存
释放后，Java程序的内存占用明显减少
2、BIO 与 NIO
原来采用BIO的架构，在读写本地文件时，我们需要从用户态切换成内核态
NIO 直接操作物理磁盘，省去了中间商赚差价
测试代码：分别使用 BIO 和 NIO 复制大文件，看看用时差别
/**
* @author shkstart shkstart@126.com
* @create 2020 0:04
*/
public class BufferTest1 {
private static final String TO = "F:\test\异界BD中字.mp4";
private static final int _100Mb = 1024 * 1024 * 100;
public static void main(String[] args) {
long sum = 0;
String src = "F:\test\异界BD中字.mp4";
for (int i = 0; i 深入 ByteBuffer 源码
ByteBuffer.allocateDirect() 方法
public static ByteBuffer allocateDirect(int capacity) {
return new DirectByteBuffer(capacity);
}DirectByteBuffer 类的构造器用到了 Unsafe 类分配本地内存
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;
}3、直接内存与 OOM
直接内存也可能导致OutofMemoryError异常。由于直接内存在Java堆外，因此它的大小不会直接受限于-Xmx指定的最大堆大小，但是系统内存是有限的，Java堆和直接内存的总和依然受限于操作系统能给出的最大内存。直接内存的缺点为：分配回收成本较高，不受JVM内存回收管理。直接内存大小可以通过MaxDirectMemorySize设置。如果不指定，默认与堆的最大值-Xmx参数值一致。
代码示例 1
/**
* 本地内存的OOM: OutOfMemoryError: Direct buffer memory
*
* @author shkstart shkstart@126.com
* @create 2020 0:09
*/
public class BufferTest2 {
private static final int BUFFER = 1024 * 1024 * 20;//20MB
public static void main(String[] args) {
ArrayList<ByteBuffer> list = new ArrayList();
int count = 0;
try {
while(true){
ByteBuffer byteBuffer = ByteBuffer.allocateDirect(BUFFER);
list.add(byteBuffer);
count++;
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} finally {
System.out.println(count);
}
}
}本地内存持续增长，直至程序抛出异常：java.lang.OutOfMemoryError: Direct buffer memory
异常信息
"C:Program FilesJavajdk1.8.0_144injava" "-javaagent:C:Program FilesJetBrainsIntelliJ IDEA 2017.3.1libidea_rt.jar=1296:C:Program FilesJetBrainsIntelliJ IDEA 2017.3.1in" -Dfile.encoding=UTF-8 -classpath "C:Program FilesJavajdk1.8.0_144jrelibcharsets.jar;C:Program FilesJavajdk1.8.0_144jrelibdeploy.jar;C:Program FilesJavajdk1.8.0_144jrelibextccess-bridge-64.jar;C:Program FilesJavajdk1.8.0_144jrelibextcldrdata.jar;C:Program FilesJavajdk1.8.0_144jrelibextdnsns.jar;C:Program FilesJavajdk1.8.0_144jrelibextjaccess.jar;C:Program FilesJavajdk1.8.0_144jrelibextjfxrt.jar;C:Program FilesJavajdk1.8.0_144jrelibextlocaledata.jar;C:Program FilesJavajdk1.8.0_144jrelibext ashorn.jar;C:Program FilesJavajdk1.8.0_144jrelibextsunec.jar;C:Program FilesJavajdk1.8.0_144jrelibextsunjce_provider.jar;C:Program FilesJavajdk1.8.0_144jrelibextsunmscapi.jar;C:Program FilesJavajdk1.8.0_144jrelibextsunpkcs11.jar;C:Program FilesJavajdk1.8.0_144jrelibextzipfs.jar;C:Program FilesJavajdk1.8.0_144jrelibjavaws.jar;C:Program FilesJavajdk1.8.0_144jrelibjce.jar;C:Program FilesJavajdk1.8.0_144jrelibjfr.jar;C:Program FilesJavajdk1.8.0_144jrelibjfxswt.jar;C:Program FilesJavajdk1.8.0_144jrelibjsse.jar;C:Program FilesJavajdk1.8.0_144jrelibmanagement-agent.jar;C:Program FilesJavajdk1.8.0_144jrelibplugin.jar;C:Program FilesJavajdk1.8.0_144jrelib esources.jar;C:Program FilesJavajdk1.8.0_144jrelib t.jar;C:UsersHeygoDesktopJVMDemooutproductionchapter11" com.atguigu.java.BufferTest2180Exception in thread "main" java.lang.OutOfMemoryError: Direct buffer memoryat java.nio.Bits.reserveMemory(Bits.java:694)at java.nio.DirectByteBuffer.<init>(DirectByteBuffer.java:123)at java.nio.ByteBuffer.allocateDirect(ByteBuffer.java:311)at com.atguigu.java.BufferTest2.main(BufferTest2.java:21)Process finished with exit code 1
代码示例 2
代码：直接通过 Unsafe 类申请本地内存
/**
* -Xmx20m -XX:MaxDirectMemorySize=10m
* @author shkstart shkstart@126.com
* @create 2020 0:36
*/
public class MaxDirectMemorySizeTest {
private static final long _1MB = 1024 * 1024;
public static void main(String[] args) throws IllegalAccessException {
Field unsafeField = Unsafe.class.getDeclaredFields()[0];
unsafeField.setAccessible(true);
Unsafe unsafe = (Unsafe)unsafeField.get(null);
while(true){
unsafe.allocateMemory(_1MB);
}
}
}JVM 参数
-Xmx20m -XX:MaxDirectMemorySize=10m
抛出 OOM 异常
"C:Program FilesJavajdk1.8.0_144injava" "-javaagent:C:Program FilesJetBrainsIntelliJ IDEA 2017.3.1libidea_rt.jar=1362:C:Program FilesJetBrainsIntelliJ IDEA 2017.3.1in" -Dfile.encoding=UTF-8 -classpath "C:Program FilesJavajdk1.8.0_144jrelibcharsets.jar;C:Program FilesJavajdk1.8.0_144jrelibdeploy.jar;C:Program FilesJavajdk1.8.0_144jrelibextccess-bridge-64.jar;C:Program FilesJavajdk1.8.0_144jrelibextcldrdata.jar;C:Program FilesJavajdk1.8.0_144jrelibextdnsns.jar;C:Program FilesJavajdk1.8.0_144jrelibextjaccess.jar;C:Program FilesJavajdk1.8.0_144jrelibextjfxrt.jar;C:Program FilesJavajdk1.8.0_144jrelibextlocaledata.jar;C:Program FilesJavajdk1.8.0_144jrelibext ashorn.jar;C:Program FilesJavajdk1.8.0_144jrelibextsunec.jar;C:Program FilesJavajdk1.8.0_144jrelibextsunjce_provider.jar;C:Program FilesJavajdk1.8.0_144jrelibextsunmscapi.jar;C:Program FilesJavajdk1.8.0_144jrelibextsunpkcs11.jar;C:Program FilesJavajdk1.8.0_144jrelibextzipfs.jar;C:Program FilesJavajdk1.8.0_144jrelibjavaws.jar;C:Program FilesJavajdk1.8.0_144jrelibjce.jar;C:Program FilesJavajdk1.8.0_144jrelibjfr.jar;C:Program FilesJavajdk1.8.0_144jrelibjfxswt.jar;C:Program FilesJavajdk1.8.0_144jrelibjsse.jar;C:Program FilesJavajdk1.8.0_144jrelibmanagement-agent.jar;C:Program FilesJavajdk1.8.0_144jrelibplugin.jar;C:Program FilesJavajdk1.8.0_144jrelib esources.jar;C:Program FilesJavajdk1.8.0_144jrelib t.jar;C:UsersHeygoDesktopJVMDemooutproductionchapter11" com.atguigu.java.MaxDirectMemorySizeTestException in thread "main" java.lang.OutOfMemoryErrorat sun.misc.Unsafe.allocateMemory(Native Method)at com.atguigu.java.MaxDirectMemorySizeTest.main(MaxDirectMemorySizeTest.java:20)Process finished with exit code 1
