What does a just-in-time JIT compiler do

Question

What does a JIT compiler specifically do as opposed to a non-JIT compiler  Can someone give a succinct and easy to understand description

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You have code that is compliled into some IL  intermediate language    When you run your program  the computer doesn t understand this code   It only understands native code   So the JIT compiler compiles your IL into native code on the fly   It does this at the method level

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20  of the byte code is used 80  of the time  The JIT compiler gets these stats and optimizes this 20  of the byte code to run faster by adding inline methods  removal of unused locks etc and also creating the bytecode specific to that machine   I am quoting from this article   I found it was handy  http   java dzone com articles just-time-compiler-jit-hotspot

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JIT stands for Just-in-Time which means that code gets compiled when it is needed  not before runtime    This is beneficial because the compiler can generate code that is optimised for your particular machine  A static compiler  like your average C compiler  will compile all of the code on to executable code on the developer s machine  Hence the compiler will perform optimisations based on some assumptions  It can compile more slowly and do more optimisations because it is not slowing execution of the program for the user

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Just In Time compiler also known as JIT compiler is used for performance improvement in Java  It is enabled by default  It is compilation done at execution time rather earlier  Java has popularized the use of JIT compiler by including it in JVM

User · Answer

JIT refers to execution engine in few of JVM implementations  one that is faster but requires more memory is a just-in-time compiler  In this scheme  the bytecodes of a method are compiled to native machine code the first time the method is invoked  The native machine code for the method is then cached  so it can be re-used the next time that same method is invoked

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JVM actually performs compilation steps during runtime for performance reasons  This means that Java doesn t have a clean compile-execution separation  It first does a so called static compilation from Java source code to bytecode  Then this bytecode is passed to the JVM for execution  But executing bytecode is slow so the JVM measures how often the bytecode is run and when it detects a  hotspot  of code that s run very frequently it performs dynamic compilation from bytecode to machinecode of the  hotspot  code  hotspot profiler   So effectively today Java programs are run by machinecode execution

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A non-JIT compiler takes source code and transforms it into machine specific byte code at compile time  A JIT compiler takes machine agnostic byte code that was generated at compile time and transforms it into machine specific byte code at run time  The JIT compiler that Java uses is what allows a single binary to run on a multitude of platforms without modification

User · Answer

In the beginning  a compiler was responsible for turning a high-level language  defined as higher level than assembler  into object code  machine instructions   which would then be linked  by a linker  into an executable   At one point in the evolution of languages  compilers would compile a high-level language into pseudo-code  which would then be interpreted  by an interpreter  to run your program   This eliminated the object code and executables  and allowed these languages to be portable to multiple operating systems and hardware platforms   Pascal  which compiled to P-Code  was one of the first  Java and C  are more recent examples   Eventually the term P-Code was replaced with bytecode  since most of the pseudo-operations are a byte long   A Just-In-Time  JIT  compiler is a feature of the run-time interpreter  that instead of interpreting bytecode every time a method is invoked  will compile the bytecode into the machine code instructions of the running machine  and then invoke this object code instead   Ideally the efficiency of running object code will overcome the inefficiency of recompiling the program every time it runs

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A just in time compiler  JIT  is a piece of software which takes receives an non executable input and returns the appropriate machine code to be executed  For example   Intermediate representation    JIT    Native machine code for the current CPU architecture       Java bytecode            --- gt         machine code      Javascript  run with V8  --- gt         machine code   The consequence of this is that for a certain CPU architecture the appropriate JIT compiler must be installed   Difference compiler  interpreter  and JIT  Although there can be exceptions in general when we want to transform source code into machine code we can use    Compiler  Takes source code and returns a executable Interpreter  Executes the program instruction by instruction  It takes an executable segment of the source code and turns that segment into machine instructions  This process is repeated until all source code is transformed into machine instructions and executed  JIT  Many different implementations of a JIT are possible  however a JIT is usually a combination of a compliler and an interpreter  The JIT first turn intermediary data  e g  Java bytecode  which it receives into machine language via interpretation  A JIT can often sense when a certain part of the code is executed often and the will compile this part for faster execution

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In the beginning  a compiler was responsible for turning a high-level language  defined as higher level than assembler  into object code  machine instructions   which would then be linked  by a linker  into an executable   At one point in the evolution of languages  compilers would compile a high-level language into pseudo-code  which would then be interpreted  by an interpreter  to run your program   This eliminated the object code and executables  and allowed these languages to be portable to multiple operating systems and hardware platforms   Pascal  which compiled to P-Code  was one of the first  Java and C  are more recent examples   Eventually the term P-Code was replaced with bytecode  since most of the pseudo-operations are a byte long   A Just-In-Time  JIT  compiler is a feature of the run-time interpreter  that instead of interpreting bytecode every time a method is invoked  will compile the bytecode into the machine code instructions of the running machine  and then invoke this object code instead   Ideally the efficiency of running object code will overcome the inefficiency of recompiling the program every time it runs

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A JIT compiler runs after the program has started and compiles the code  usually bytecode or some kind of VM instructions  on the fly  or just-in-time  as it s called  into a form that s usually faster  typically the host CPU s native instruction set  A JIT has access to dynamic runtime information whereas a standard compiler doesn t and can make better optimizations like inlining functions that are used frequently   This is in contrast to a traditional compiler that compiles all the code to machine language before the program is first run   To paraphrase  conventional compilers build the whole program as an EXE file BEFORE the first time you run it  For newer style programs  an assembly is generated with pseudocode  p-code   Only AFTER you execute the program on the OS  e g   by double-clicking on its icon  will the  JIT  compiler kick in and generate machine code  m-code  that the Intel-based processor or whatever will understand

User · Answer

You have code that is compliled into some IL  intermediate language    When you run your program  the computer doesn t understand this code   It only understands native code   So the JIT compiler compiles your IL into native code on the fly   It does this at the method level

User · Answer

A non-JIT compiler takes source code and transforms it into machine specific byte code at compile time  A JIT compiler takes machine agnostic byte code that was generated at compile time and transforms it into machine specific byte code at run time  The JIT compiler that Java uses is what allows a single binary to run on a multitude of platforms without modification

User · Answer

Just In Time Compiler  JIT    It compiles the java bytecodes into machine instructions of that specific CPU    For example  if we have a loop statement in our java code    while i lt 10                  a a i                  The above loop code runs for 10 times if the value of i is 0   It is not necessary to compile the bytecode for 10 times again and again as the same instruction is going to execute for 10 times  In that case  it is necessary to compile that code only once and the value can be changed for the required number of times  So  Just In Time  JIT  Compiler keeps track of such statements and methods  as said above before  and compiles such pieces of byte code into machine code for better performance    Another similar example   is that a search for a pattern using  Regular Expression  in a list of strings sentences   JIT Compiler doesn t compile all the code to machine code  It compiles code that have a similar pattern at run time   See this Oracle documentation on Understand JIT to read more

User · Answer

In the beginning  a compiler was responsible for turning a high-level language  defined as higher level than assembler  into object code  machine instructions   which would then be linked  by a linker  into an executable   At one point in the evolution of languages  compilers would compile a high-level language into pseudo-code  which would then be interpreted  by an interpreter  to run your program   This eliminated the object code and executables  and allowed these languages to be portable to multiple operating systems and hardware platforms   Pascal  which compiled to P-Code  was one of the first  Java and C  are more recent examples   Eventually the term P-Code was replaced with bytecode  since most of the pseudo-operations are a byte long   A Just-In-Time  JIT  compiler is a feature of the run-time interpreter  that instead of interpreting bytecode every time a method is invoked  will compile the bytecode into the machine code instructions of the running machine  and then invoke this object code instead   Ideally the efficiency of running object code will overcome the inefficiency of recompiling the program every time it runs

User · Answer

Jit stands for just in time compiler jit is a program that turns java byte code into instruction that can be sent directly to the processor   Using the java just in time compiler  really a second compiler  at the particular system platform  complies the bytecode into particular system code once the code has been re-compiled by the jit complier  it will usually run more quickly in the computer      The just-in-time compiler comes with the virtual machine and is used optionally  It compiles the bytecode into platform-specific executable code that is immediately executed

User · Answer

JIT stands for Just-in-Time which means that code gets compiled when it is needed  not before runtime    This is beneficial because the compiler can generate code that is optimised for your particular machine  A static compiler  like your average C compiler  will compile all of the code on to executable code on the developer s machine  Hence the compiler will perform optimisations based on some assumptions  It can compile more slowly and do more optimisations because it is not slowing execution of the program for the user

User · Answer

JIT-Just in time the word itself says when it s needed  on demand   Typical scenario   The source code is completely converted into machine code  JIT scenario   The source code will be converted into assembly language like structure  for ex IL  intermediate language  for C   ByteCode for java    The intermediate code is converted into machine language only when the application needs that is required codes are only converted to machine code   JIT vs Non-JIT comparison    In JIT not all the code is converted into machine code first a part of the code that is necessary will be converted into machine code then if a method or functionality called is not in machine then that will be turned into machine code    it reduces burden on the CPU  As the machine code will be generated on run time    the JIT compiler will produce machine code that is optimised for running machine s CPU architecture    JIT Examples    In Java JIT is in JVM  Java Virtual Machine   In C  it is in CLR  Common Language Runtime  In Android it is in DVM  Dalvik Virtual Machine   or ART  Android RunTime  in newer versions

User · Answer

In the beginning  a compiler was responsible for turning a high-level language  defined as higher level than assembler  into object code  machine instructions   which would then be linked  by a linker  into an executable   At one point in the evolution of languages  compilers would compile a high-level language into pseudo-code  which would then be interpreted  by an interpreter  to run your program   This eliminated the object code and executables  and allowed these languages to be portable to multiple operating systems and hardware platforms   Pascal  which compiled to P-Code  was one of the first  Java and C  are more recent examples   Eventually the term P-Code was replaced with bytecode  since most of the pseudo-operations are a byte long   A Just-In-Time  JIT  compiler is a feature of the run-time interpreter  that instead of interpreting bytecode every time a method is invoked  will compile the bytecode into the machine code instructions of the running machine  and then invoke this object code instead   Ideally the efficiency of running object code will overcome the inefficiency of recompiling the program every time it runs

User · Answer

You have code that is compliled into some IL  intermediate language    When you run your program  the computer doesn t understand this code   It only understands native code   So the JIT compiler compiles your IL into native code on the fly   It does this at the method level

User · Answer

After the byte code  which is architecture neutral  has been generated by the Java compiler  the execution will be handled by the JVM  in Java   The byte code will be loaded in to JVM by the loader and then each byte instruction is interpreted   When we need to call a method multiple times  we need to interpret the same code many times and this may take more time than is needed  So we have the JIT  just-in-time  compilers  When the byte has been is loaded in to JVM  its run time   the whole code will be compiled rather than interpreted  thus saving time   JIT compilers works only during run time  so we do not have any binary output

User · Answer

JIT compiler only compiles the byte-code to equivalent native code at first execution  Upon every successive execution  the JVM merely uses the already compiled native code to optimize performance     Without JIT compiler  the JVM interpreter translates the byte-code line-by-line to make it appear as if a native application is being executed     Source

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I know this is an old thread  but runtime optimization is another important part of JIT compilation that doesn t seemed to be discussed here   Basically  the JIT compiler can monitor the program as it runs to determine ways to improve execution   Then  it can make those changes on the fly - during runtime   Google JIT optimization  javaworld has a pretty good article about it

User · Answer

After the byte code  which is architecture neutral  has been generated by the Java compiler  the execution will be handled by the JVM  in Java   The byte code will be loaded in to JVM by the loader and then each byte instruction is interpreted   When we need to call a method multiple times  we need to interpret the same code many times and this may take more time than is needed  So we have the JIT  just-in-time  compilers  When the byte has been is loaded in to JVM  its run time   the whole code will be compiled rather than interpreted  thus saving time   JIT compilers works only during run time  so we do not have any binary output

User · Answer

A non-JIT compiler takes source code and transforms it into machine specific byte code at compile time  A JIT compiler takes machine agnostic byte code that was generated at compile time and transforms it into machine specific byte code at run time  The JIT compiler that Java uses is what allows a single binary to run on a multitude of platforms without modification

User · Answer

JIT refers to execution engine in few of JVM implementations  one that is faster but requires more memory is a just-in-time compiler  In this scheme  the bytecodes of a method are compiled to native machine code the first time the method is invoked  The native machine code for the method is then cached  so it can be re-used the next time that same method is invoked

User · Answer

As other have mentioned  JIT stands for Just-in-Time which means that code gets compiled when it is needed  not before runtime    Just to add a point to above discussion JVM maintains a count as of how many time a function is executed  If this count exceeds a predefined limit JIT compiles the code into machine language which can directly be executed by the processor  unlike the normal case in which javac compile the code into bytecode and then java - the interpreter interprets this bytecode line by line converts it into machine code and executes    Also next time this function is calculated same compiled code is executed again unlike normal interpretation in which the code is interpreted again line by line  This makes execution faster

User · Answer

Just In Time compiler also known as JIT compiler is used for performance improvement in Java  It is enabled by default  It is compilation done at execution time rather earlier  Java has popularized the use of JIT compiler by including it in JVM

User · Answer

A non-JIT compiler takes source code and transforms it into machine specific byte code at compile time  A JIT compiler takes machine agnostic byte code that was generated at compile time and transforms it into machine specific byte code at run time  The JIT compiler that Java uses is what allows a single binary to run on a multitude of platforms without modification

User · Answer

A JIT compiler runs after the program has started and compiles the code  usually bytecode or some kind of VM instructions  on the fly  or just-in-time  as it s called  into a form that s usually faster  typically the host CPU s native instruction set  A JIT has access to dynamic runtime information whereas a standard compiler doesn t and can make better optimizations like inlining functions that are used frequently   This is in contrast to a traditional compiler that compiles all the code to machine language before the program is first run   To paraphrase  conventional compilers build the whole program as an EXE file BEFORE the first time you run it  For newer style programs  an assembly is generated with pseudocode  p-code   Only AFTER you execute the program on the OS  e g   by double-clicking on its icon  will the  JIT  compiler kick in and generate machine code  m-code  that the Intel-based processor or whatever will understand

User · Answer

just-in-time  JIT  compilation   also dynamic translation or run-time compilation   is a way of executing computer code that involves compilation during execution of a program     at run time     rather than prior to execution   IT compilation is a combination of the two traditional approaches to translation to machine code     ahead-of-time compilation  AOT   and interpretation     and combines some advantages and drawbacks of both  JIT compilation combines the speed of compiled code with the flexibility of interpretation   Let s consider JIT used in JVM   For example  the HotSpot JVM JIT compilers generate dynamic optimizations  In other words  they make optimization decisions while the Java application is running and generate high-performing native machine instructions targeted for the underlying system architecture   When a method is chosen for compilation  the JVM feeds its bytecode to the Just-In-Time compiler  JIT   The JIT needs to understand the semantics and syntax of the bytecode before it can compile the method correctly  To help the JIT compiler analyze the method  its bytecode are first reformulated in an internal representation called trace trees  which resembles machine code more closely than bytecode  Analysis and optimizations are then performed on the trees of the method  At the end  the trees are translated into native code      A trace tree is a data structure that is used in the runtime compilation of programming code  Trace trees are used in a type of  just in time compiler  that traces code executing during hotspots and compiles it  Refer this    Refer      http   www oracle com webfolder technetwork tutorials obe java gc01 index html https   en wikipedia org wiki Just-in-time compilation

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just-in-time  JIT  compilation   also dynamic translation or run-time compilation   is a way of executing computer code that involves compilation during execution of a program     at run time     rather than prior to execution   IT compilation is a combination of the two traditional approaches to translation to machine code     ahead-of-time compilation  AOT   and interpretation     and combines some advantages and drawbacks of both  JIT compilation combines the speed of compiled code with the flexibility of interpretation   Let s consider JIT used in JVM   For example  the HotSpot JVM JIT compilers generate dynamic optimizations  In other words  they make optimization decisions while the Java application is running and generate high-performing native machine instructions targeted for the underlying system architecture   When a method is chosen for compilation  the JVM feeds its bytecode to the Just-In-Time compiler  JIT   The JIT needs to understand the semantics and syntax of the bytecode before it can compile the method correctly  To help the JIT compiler analyze the method  its bytecode are first reformulated in an internal representation called trace trees  which resembles machine code more closely than bytecode  Analysis and optimizations are then performed on the trees of the method  At the end  the trees are translated into native code      A trace tree is a data structure that is used in the runtime compilation of programming code  Trace trees are used in a type of  just in time compiler  that traces code executing during hotspots and compiles it  Refer this    Refer      http   www oracle com webfolder technetwork tutorials obe java gc01 index html https   en wikipedia org wiki Just-in-time compilation

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A JIT compiler runs after the program has started and compiles the code  usually bytecode or some kind of VM instructions  on the fly  or just-in-time  as it s called  into a form that s usually faster  typically the host CPU s native instruction set  A JIT has access to dynamic runtime information whereas a standard compiler doesn t and can make better optimizations like inlining functions that are used frequently   This is in contrast to a traditional compiler that compiles all the code to machine language before the program is first run   To paraphrase  conventional compilers build the whole program as an EXE file BEFORE the first time you run it  For newer style programs  an assembly is generated with pseudocode  p-code   Only AFTER you execute the program on the OS  e g   by double-clicking on its icon  will the  JIT  compiler kick in and generate machine code  m-code  that the Intel-based processor or whatever will understand

User · Answer

JIT stands for Just-in-Time which means that code gets compiled when it is needed  not before runtime    This is beneficial because the compiler can generate code that is optimised for your particular machine  A static compiler  like your average C compiler  will compile all of the code on to executable code on the developer s machine  Hence the compiler will perform optimisations based on some assumptions  It can compile more slowly and do more optimisations because it is not slowing execution of the program for the user

User · Answer

You have code that is compliled into some IL  intermediate language    When you run your program  the computer doesn t understand this code   It only understands native code   So the JIT compiler compiles your IL into native code on the fly   It does this at the method level

User · Answer

JIT stands for Just-in-Time which means that code gets compiled when it is needed  not before runtime    This is beneficial because the compiler can generate code that is optimised for your particular machine  A static compiler  like your average C compiler  will compile all of the code on to executable code on the developer s machine  Hence the compiler will perform optimisations based on some assumptions  It can compile more slowly and do more optimisations because it is not slowing execution of the program for the user

User · Answer

A JIT compiler runs after the program has started and compiles the code  usually bytecode or some kind of VM instructions  on the fly  or just-in-time  as it s called  into a form that s usually faster  typically the host CPU s native instruction set  A JIT has access to dynamic runtime information whereas a standard compiler doesn t and can make better optimizations like inlining functions that are used frequently   This is in contrast to a traditional compiler that compiles all the code to machine language before the program is first run   To paraphrase  conventional compilers build the whole program as an EXE file BEFORE the first time you run it  For newer style programs  an assembly is generated with pseudocode  p-code   Only AFTER you execute the program on the OS  e g   by double-clicking on its icon  will the  JIT  compiler kick in and generate machine code  m-code  that the Intel-based processor or whatever will understand

User · Answer

JIT compiler only compiles the byte-code to equivalent native code at first execution  Upon every successive execution  the JVM merely uses the already compiled native code to optimize performance     Without JIT compiler  the JVM interpreter translates the byte-code line-by-line to make it appear as if a native application is being executed     Source

User · Answer

JIT-Just in time the word itself says when it s needed  on demand   Typical scenario   The source code is completely converted into machine code  JIT scenario   The source code will be converted into assembly language like structure  for ex IL  intermediate language  for C   ByteCode for java    The intermediate code is converted into machine language only when the application needs that is required codes are only converted to machine code   JIT vs Non-JIT comparison    In JIT not all the code is converted into machine code first a part of the code that is necessary will be converted into machine code then if a method or functionality called is not in machine then that will be turned into machine code    it reduces burden on the CPU  As the machine code will be generated on run time    the JIT compiler will produce machine code that is optimised for running machine s CPU architecture    JIT Examples    In Java JIT is in JVM  Java Virtual Machine   In C  it is in CLR  Common Language Runtime  In Android it is in DVM  Dalvik Virtual Machine   or ART  Android RunTime  in newer versions

User · Answer

As other have mentioned  JIT stands for Just-in-Time which means that code gets compiled when it is needed  not before runtime    Just to add a point to above discussion JVM maintains a count as of how many time a function is executed  If this count exceeds a predefined limit JIT compiles the code into machine language which can directly be executed by the processor  unlike the normal case in which javac compile the code into bytecode and then java - the interpreter interprets this bytecode line by line converts it into machine code and executes    Also next time this function is calculated same compiled code is executed again unlike normal interpretation in which the code is interpreted again line by line  This makes execution faster

User · Answer

Jit stands for just in time compiler jit is a program that turns java byte code into instruction that can be sent directly to the processor   Using the java just in time compiler  really a second compiler  at the particular system platform  complies the bytecode into particular system code once the code has been re-compiled by the jit complier  it will usually run more quickly in the computer      The just-in-time compiler comes with the virtual machine and is used optionally  It compiles the bytecode into platform-specific executable code that is immediately executed

User · Answer

A just in time compiler  JIT  is a piece of software which takes receives an non executable input and returns the appropriate machine code to be executed  For example   Intermediate representation    JIT    Native machine code for the current CPU architecture       Java bytecode            --- gt         machine code      Javascript  run with V8  --- gt         machine code   The consequence of this is that for a certain CPU architecture the appropriate JIT compiler must be installed   Difference compiler  interpreter  and JIT  Although there can be exceptions in general when we want to transform source code into machine code we can use    Compiler  Takes source code and returns a executable Interpreter  Executes the program instruction by instruction  It takes an executable segment of the source code and turns that segment into machine instructions  This process is repeated until all source code is transformed into machine instructions and executed  JIT  Many different implementations of a JIT are possible  however a JIT is usually a combination of a compliler and an interpreter  The JIT first turn intermediary data  e g  Java bytecode  which it receives into machine language via interpretation  A JIT can often sense when a certain part of the code is executed often and the will compile this part for faster execution

User · Answer

Just In Time Compiler  JIT    It compiles the java bytecodes into machine instructions of that specific CPU    For example  if we have a loop statement in our java code    while i lt 10                  a a i                  The above loop code runs for 10 times if the value of i is 0   It is not necessary to compile the bytecode for 10 times again and again as the same instruction is going to execute for 10 times  In that case  it is necessary to compile that code only once and the value can be changed for the required number of times  So  Just In Time  JIT  Compiler keeps track of such statements and methods  as said above before  and compiles such pieces of byte code into machine code for better performance    Another similar example   is that a search for a pattern using  Regular Expression  in a list of strings sentences   JIT Compiler doesn t compile all the code to machine code  It compiles code that have a similar pattern at run time   See this Oracle documentation on Understand JIT to read more

User · Answer

I know this is an old thread  but runtime optimization is another important part of JIT compilation that doesn t seemed to be discussed here   Basically  the JIT compiler can monitor the program as it runs to determine ways to improve execution   Then  it can make those changes on the fly - during runtime   Google JIT optimization  javaworld has a pretty good article about it

User · Answer

JVM actually performs compilation steps during runtime for performance reasons  This means that Java doesn t have a clean compile-execution separation  It first does a so called static compilation from Java source code to bytecode  Then this bytecode is passed to the JVM for execution  But executing bytecode is slow so the JVM measures how often the bytecode is run and when it detects a  hotspot  of code that s run very frequently it performs dynamic compilation from bytecode to machinecode of the  hotspot  code  hotspot profiler   So effectively today Java programs are run by machinecode execution

User · Answer

20  of the byte code is used 80  of the time  The JIT compiler gets these stats and optimizes this 20  of the byte code to run faster by adding inline methods  removal of unused locks etc and also creating the bytecode specific to that machine   I am quoting from this article   I found it was handy  http   java dzone com articles just-time-compiler-jit-hotspot

[compilation] What does a just-in-time (JIT) compiler do?

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