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The two approaches differ in where you define that code.")]),t._v(" "),a("p",[t._v("In Java, a thread is represented by an object - an instance of "),a("a",{attrs:{href:"https://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("java.lang.Thread"),a("OutboundLink")],1),t._v(" or its subclass. 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You can only add a "),a("a",{attrs:{href:"https://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("Thread"),a("OutboundLink")],1),t._v(" to a "),a("a",{attrs:{href:"https://docs.oracle.com/javase/8/docs/api/java/lang/ThreadGroup.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("ThreadGroup"),a("OutboundLink")],1),t._v(" using a "),a("a",{attrs:{href:"https://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("Thread"),a("OutboundLink")],1),t._v("'s constructor. 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punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("h2",{attrs:{id:"using-threadlocal"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#using-threadlocal"}},[t._v("#")]),t._v(" Using ThreadLocal")]),t._v(" "),a("p",[t._v("A useful tool in Java Concurrency is "),a("code",[t._v("ThreadLocal")]),t._v(" – this allows you to have a variable that will be unique to a given thread. Thus, if the same code runs in different threads, these executions will not share the value, but instead each thread has its own variable that is "),a("strong",[t._v("local to the thread")]),t._v(".")]),t._v(" "),a("p",[t._v("For example, this is frequently used to establish the context (such as authorization information) of handling a request in a servlet. You might do something like this:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("private")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("static")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("final")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ThreadLocal")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("MyUserContext")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),t._v(" contexts "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ThreadLocal")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("static")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("MyUserContext")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getContext")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" contexts"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("get")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// get returns the variable unique to this thread")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("doGet")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("MyUserContext")]),t._v(" context "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("magicGetContextFromRequest")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("request"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" \n    contexts"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("put")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("context"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// save that context to our thread-local - other threads")]),t._v("\n                           "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// making this call don't overwrite ours")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("try")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// business logic")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("finally")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        contexts"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("remove")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 'ensure' removal of thread-local variable")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("Now, instead of passing "),a("code",[t._v("MyUserContext")]),t._v(" into every single method, you can instead use "),a("code",[t._v("MyServlet.getContext()")]),t._v(" where you need it. Now of course, this does introduce a variable that needs to be documented, but it’s thread-safe, which eliminates a lot of the downsides to using such a highly scoped variable.")]),t._v(" "),a("p",[t._v("The key advantage here is that every thread has its own thread local variable in that "),a("code",[t._v("contexts")]),t._v(" container. As long as you use it from a defined entry point (like demanding that each servlet maintains its context, or perhaps by adding a servlet filter) you can rely on this context being there when you need it.")]),t._v(" "),a("h2",{attrs:{id:"visualizing-read-write-barriers-while-using-synchronized-volatile"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#visualizing-read-write-barriers-while-using-synchronized-volatile"}},[t._v("#")]),t._v(" Visualizing read/write barriers while using synchronized / volatile")]),t._v(" "),a("p",[t._v("As we know that we should use "),a("code",[t._v("synchronized")]),t._v(" keyword to make execution of a method or block exclusive. But few of us may not be aware of one more important aspect of using "),a("code",[t._v("synchronized")]),t._v(" and "),a("code",[t._v("volatile")]),t._v(" keyword: "),a("strong",[t._v("apart from making a unit of code atomic, it also provides read / write barrier")]),t._v(". What is this read / write barrier? Let's discuss this using an example:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("class")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Counter")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n\n  "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("private")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Integer")]),t._v(" count "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("10")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n  "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("synchronized")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("incrementCount")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    count"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("++")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n  "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Integer")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getCount")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" count"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("Let's suppose a thread "),a("strong",[t._v("A")]),t._v(" calls "),a("code",[t._v("incrementCount()")]),t._v(" first then another thread "),a("strong",[t._v("B")]),t._v(" calls "),a("code",[t._v("getCount()")]),t._v(". In this scenario there is no guarantee that B will see updated value of "),a("code",[t._v("count")]),t._v(". It may still see "),a("code",[t._v("count")]),t._v(" as "),a("code",[t._v("10")]),t._v(", even it is also possible that it never sees updated value of "),a("code",[t._v("count")]),t._v(" ever.")]),t._v(" "),a("p",[t._v("To understand this behavior we need to understand how Java memory model integrates with hardware architecture. In Java, each thread has it's own thread stack. This stack contains: method call stack and local variable created in that thread. In a multi core system, it is quite possible that two threads are running concurrently in separate cores. In such scenario it is possible that part of a thread's stack lies inside register / cache of a core. If inside a thread, an object is accessed using "),a("code",[t._v("synchronized")]),t._v(" (or "),a("code",[t._v("volatile")]),t._v(") keyword, after "),a("code",[t._v("synchronized")]),t._v(" block that thread syncs it's local copy of that variable with the main memory. This creates a read / write barrier and makes sure that the thread is seeing the latest value of that object.")]),t._v(" "),a("p",[t._v("But in our case, since thread B has not used synchronized access to "),a("code",[t._v("count")]),t._v(", it might be refering value of "),a("code",[t._v("count")]),t._v(" stored in register and may never see updates from thread A. To make sure that B sees latest value of count we need to make "),a("code",[t._v("getCount()")]),t._v(" synchronized as well.")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("synchronized")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Integer")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getCount")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" count"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("Now when thread A is done with updating "),a("code",[t._v("count")]),t._v(" it unlocks "),a("code",[t._v("Counter")]),t._v(" instance, at the same time creates write barrier and flushes all changes done inside that block to the main memory. Similarly when thread B acquires lock on the same instance of "),a("code",[t._v("Counter")]),t._v(", it enters into read barrier and reads value of "),a("code",[t._v("count")]),t._v(" from main memory and sees all updates.")]),t._v(" "),a("p",[a("a",{attrs:{href:"https://i.stack.imgur.com/tnFLB.png",target:"_blank",rel:"noopener noreferrer"}},[a("img",{attrs:{src:"https://i.stack.imgur.com/tnFLB.png",alt:"visibility"}}),a("OutboundLink")],1)]),t._v(" "),a("p",[t._v("Same visibility effect goes for "),a("code",[t._v("volatile")]),t._v(" read / writes as well. All variables updated prior to write to "),a("code",[t._v("volatile")]),t._v(" will be flushed to main memory and all reads after "),a("code",[t._v("volatile")]),t._v(" variable read will be from main memory.")]),t._v(" "),a("h2",{attrs:{id:"multiple-producer-consumer-example-with-shared-global-queue"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#multiple-producer-consumer-example-with-shared-global-queue"}},[t._v("#")]),t._v(" Multiple producer/consumer example with shared global queue")]),t._v(" "),a("p",[t._v("Below code showcases multiple Producer/Consumer program. Both Producer and Consumer threads share same global queue.")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("import")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token namespace"}},[t._v("java"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("util"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("concurrent"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")])]),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("*")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("import")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token namespace"}},[t._v("java"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("util"),a("span",{pre:!0,attrs:{class:"token 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In practice, there won't be any speedup by using threads for a task this small.  A slowdown is likely, since the overheads of task creation and scheduling will swamp the time taken to run a task.\n")]),t._v(" "),a("li",[t._v("\nIf you were using Java 7 and earlier, you would use anonymous classes instead of lambdas to implement the tasks.\n")]),t._v(" "),a("h2",{attrs:{id:"thread-interruption-stopping-threads"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#thread-interruption-stopping-threads"}},[t._v("#")]),t._v(" Thread Interruption / Stopping Threads")]),t._v(" "),a("p",[t._v("Each Java Thread has an interrupt flag, which is initially false. Interrupting a thread, is essentially nothing more than setting that flag to true. The code running on that thread can check the flag on occasion and act upon it. The code can also ignore it completely.\nBut why would each Thread have such a flag? After all, having a boolean flag on a thread is something we can just organize ourselves, if and when we need it. Well, there are methods that behave in a special way when the thread they're running on is interrupted. These methods are called blocking methods. These are methods that put the thread in the WAITING or TIMED_WAITING state. When a thread is in this state, interrupting it, will cause an InterruptedException to be thrown on the interrupted thread, rather than the interrupt flag being set to true, and the thread becomes RUNNABLE again. Code that invokes a blocking method is forced to deal with the InterruptedException, since it is a checked exception. So, and hence its name, an interrupt can have the effect of interrupting a WAIT, effectively ending it. Note that not all methods that are somehow waiting (e.g. blocking IO) respond to interruption in that way, as they don't put the thread in a waiting state.\nLastly a thread that has its interrupt flag set, that enters a blocking method (i.e. tries to get into a waiting state), will immediately throw an InterruptedException and the interrupt flag will be cleared.")]),t._v(" "),a("p",[t._v("Other than these mechanics, Java does not assign any special semantic meaning to interruption. Code is free to interpret an interrupt any way it likes. But most often interruption is used to signal to a thread it should stop running at its earliest convenience. But, as should be clear from the above, it is up to the code on that thread to react to that interruption appropriately in order to stop running. Stopping a thread is a collaboration.\nWhen a thread is interrupted its running code can be several levels deep into the stacktrace. Most code doesn't call a blocking method, and finishes timely enough to not delay the stopping of the thread unduly. The code that should mostly be concerned with being responsive to interruption, is code that is in a loop handling tasks until there are none left, or until a flag is set signalling it to stop that loop. Loops that handle possibly infinite tasks (i.e. they keep running in principle) should check the interrupt flag in order to exit the loop. For finite loops the semantics may dictate that all tasks must be finished before ending, or it may be appropriate to leave some tasks unhandled.\nCode that calls blocking methods will be forced to deal with the InterruptedException. If at all semantically possible, it can simply propagate the InterruptedException and declare to throw it. As such it becomes a blocking method itself in regard to its callers. If it cannot propagate the exception, it should at the very least set the interrupted flag, so callers higher up the stack also know the thread was interrupted. In some cases the method needs to continue waiting regardless of the InterruptedException, in which case it must delay setting the interrupted flag until after it is done waiting, this may involve setting a local variable, which is to be checked prior to exiting the method to then interrupt its thread.")]),t._v(" "),a("p",[a("strong",[t._v("Examples :")])]),t._v(" "),a("p",[a("strong",[t._v("Example of code that stops handling tasks upon interruption")])]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("class")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("TaskHandler")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("implements")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Runnable")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    \n    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