transmittable-thread-local
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alibaba
fix:add synchronized to weakHashMap,avoid endless loop while updating…
在更新ExtensionTransformlets时使用到了WeakHashMap，在并发时，WeakHashMap在扩容的时候有一定概率形成环形链表，导致死循环，CPU飙升。
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在更新ExtensionTransformlets时使用到了WeakHashMap， 在并发时，WeakHashMap在扩容的时候有一定概率形成环形链表，导致死循环，CPU飙升。
@dingmingcheng 你有实际出现/复现『在并发时，WeakHashMap死循环CPU飙升』风险的情况吗？
类加载过程（即java.lang.instrument.ClassFileTransformer#transform调用） 应该是 『等效单线程』不并发的。
上面这点需要通过JVM官方文档来确认的。
可以帮忙一起确认吗？ ♥️ @dingmingcheng
如果 类加载过程 是 『等效单线程』的，则不需要 上面的Collections.synchronizedMap的处理逻辑。
# 也规避了实现逻辑可以不必要的复杂性。
@oldratlee 有出现过。但只有当时dump下来的线程栈信息，有两个线程都卡在了下面的代码。Java版本是1.8.0_192。
"http-nio-127.0.0.1-8300-exec-97" #195 daemon prio=5 os_prio=0 tid=0x00007f03513a6000 nid=0x4e6 runnable [0x00007f02eab59000]
java.lang.Thread.State: RUNNABLE
at java.util.WeakHashMap.getEntry(WeakHashMap.java:431)
at java.util.WeakHashMap.containsKey(WeakHashMap.java:417)
at com.alibaba.ttl.threadpool.agent.TtlExtensionTransformletManager.collectExtensionTransformlet(TtlExtensionTransformletManager.java:67)
at com.alibaba.ttl.threadpool.agent.TtlTransformer.transform(TtlTransformer.java:68)
at sun.instrument.TransformerManager.transform(TransformerManager.java:188)
at sun.instrument.InstrumentationImpl.transform(InstrumentationImpl.java:428)
at sun.misc.Unsafe.defineClass(Native Method)
at sun.reflect.ClassDefiner.defineClass(ClassDefiner.java:63)
at sun.reflect.MethodAccessorGenerator$1.run(MethodAccessorGenerator.java:399)
at sun.reflect.MethodAccessorGenerator$1.run(MethodAccessorGenerator.java:394)
at java.security.AccessController.doPrivileged(Native Method)
at sun.reflect.MethodAccessorGenerator.generate(MethodAccessorGenerator.java:393)
at sun.reflect.MethodAccessorGenerator.generateMethod(MethodAccessorGenerator.java:75)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:53)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:689)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:755)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178)