y987425112 opened a new pull request #145: Adding volatile keywords to member variables URL: https://github.com/apache/tomcat/pull/145 The semantics of the Java programming language allow compilers and microprocessors to perform optimizations that can interact with incorrectly synchronized code in ways that can produce behaviors that seem paradoxical.[https://edelivery.oracle.com/otn-pub/jcp/memory_model-1.0-pfd-spec-oth-JSpec/memory_model-1_0-pfd-spec.pdf](url) page6 For example ``` org.apache.catalina.core.StandardContext.java private String applicationListeners[] = new String[0]; ......... @Override public void addApplicationListener(String listener) { ``` synchronized (applicationListenersLock) { String results[] = new String[applicationListeners.length + 1]; for (int i = 0; i < applicationListeners.length; i++) { if (listener.equals(applicationListeners[i])) { log.info(sm.getString("standardContext.duplicateListener",listener)); return; } results[i] = applicationListeners[i]; } results[applicationListeners.length] = listener; applicationListeners = results; } fireContainerEvent("addApplicationListener", listener); } We expect this line of code(`applicationListeners = results;`) to be executed eventually But because of instruction reordering,The following operating procedures are also allowed to exis ``` @Override public void addApplicationListener(String listener) { ``` synchronized (applicationListenersLock) { String results[] = new String[applicationListeners.length + 1]; applicationListeners = results; for (int i = 0; i < applicationListeners.length; i++) { if (listener.equals(applicationListeners[i])) { log.info(sm.getString("standardContext.duplicateListener",listener)); return; } results[i] = applicationListeners[i]; } results[applicationListeners.length] = listener; } fireContainerEvent("addApplicationListener", listener); } If this happens, it will be different from our expectations. If volatile modifiers are added to variables, instruction reordering is prohibited `private String applicationListeners[] = new String[0]; `--> ` private volatile String applicationListeners[] = new String[0];` Satisfy the purpose of the last execution of this line of code(`applicationListeners = results;`).
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