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这几天使用MappedByteBuffer对系统进行了实现，在压测过程中发现启动多个group订阅时服务端会发生jvm crash ，排查后找到了原因，记录一下。

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部分信息如下：

#

# A fatal error has been detected by the Java Runtime Environment:

#

#? EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x01b8d4d7, pid=10236, tid=8452

#

# JRE version: 6.0_18-b02

# Java VM: Java HotSpot(TM) Client VM (16.0-b09 mixed mode, sharing windows-x86 )

# Problematic frame:

# J? java.nio.DirectByteBuffer.get(I)B

#

# An error report file with more information is saved as:

# D:\workspace_new\metamorphosis\metamorphosis-server\hs_err_pid10236.log

#

# If you would like to submit a bug report, please visit:

#?? http://java.sun.com/webapps/bugreport/crash.jsp

#

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原因是在MappedByteBuffer释放后再对它进行读操作的话就会引发这个crash，在并发情况下很容易正在释放时另一个线程正开始读取，于是crash就发生了。下面这个测试代码可以重现：

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@Test

public void unmapAndRead()throws Exception {

? File file =new File("unmapAndRead.txt");

? FileChannel fileChannel =new RandomAccessFile(file,"rw").getChannel();

? MappedByteBuffer buf = fileChannel.map(MapMode.READ_WRITE, 0, 4096);

? buf.putInt(5);

? buf.force();