1、直接内存概述
直接内存
不是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。
直接内存是在Java堆外的、直接向系统申请的内存区间。
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。即读写性能高。
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
非直接缓存区（BIO）
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
直接缓冲区（NIO）
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 < 3; i++) {
String dest = "F:\\test\\异界BD中字_" + i + ".mp4";
// sum += io(src,dest);//54606
sum += directBuffer(src, dest);//50244
}
System.out.println("总花费的时间为：" + sum);
}
private static long directBuffer(String src, String dest) {
long start = System.currentTimeMillis();
FileChannel inChannel = null;
FileChannel outChannel = null;
try {
inChannel = new FileInputStream(src).getChannel();
outChannel = new FileOutputStream(dest).getChannel();
ByteBuffer byteBuffer = ByteBuffer.allocateDirect(_100Mb);
while (inChannel.read(byteBuffer) != -1) {
byteBuffer.flip();//修改为读数据模式
outChannel.write(byteBuffer);
byteBuffer.clear();//清空
}
} catch (IOException e) {
e.printStackTrace();
} finally {
if (inChannel != null) {
try {
inChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (outChannel != null) {
try {
outChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
long end = System.currentTimeMillis();
return end - start;
}
private static long io(String src, String dest) {
long start = System.currentTimeMillis();
FileInputStream fis = null;
FileOutputStream fos = null;
try {
fis = new FileInputStream(src);
fos = new FileOutputStream(dest);
byte[] buffer = new byte[_100Mb];
while (true) {
int len = fis.read(buffer);
if (len == -1) {
break;
}
fos.write(buffer, 0, len);
}
} catch (IOException e) {
e.printStackTrace();
} finally {
if (fis != null) {
try {
fis.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fos != null) {
try {
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
long end = System.currentTimeMillis();
return end - start;
}
}
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
/**
* 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 Files\Java\jdk1.8.0_144\bin\java" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\lib\idea_rt.jar=1296:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_144\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\rt.jar;C:\Users\Heygo\Desktop\JVMDemo\out\production\chapter11" com.atguigu.java.BufferTest2
180
Exception in thread "main" java.lang.OutOfMemoryError: Direct buffer memory
at 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