什么是JIT?
JIT是just in time,即时编译技术。使用该技术,能够加速java程序的执行速度。下面,就对该技术做个简单的讲解。
首先,我们大家都知道,通常javac将程序源代码编译,转换成java字节码,JVM通过解释字节码将其翻译成对应的机器指令,逐条读入,逐条解释翻译。很显然,经过解释执行,其执行速度必然会比可执行的二进制字节码程序慢。为了提高执行速度,引入了JIT技术。
在运行时JIT会把翻译过的机器码保存起来,已备下次使用,因此从理论上来说,采用该JIT技术可以,可以接近以前纯编译技术。下面我看看,JIT的工作过程。
JIT 编译过程
当JIT编译启用时(默认是启用的),JVM读入.class文件解释后,将其发给JIT编译器。JIT编译器将字节码编译成本机机器代码,下图展示了该过程。
通过上面的解释,我们了解了JIT的工作原理及过程,同样也发现了个问题,由于JIT对每条字节码都进行编译,造成了编译过程负担过重。为了避免这种情况,当前的JIT只对经常执行的字节码进行编译,如循环等。
需要说明的是,JIT并不总是奏效,不能期望JIT一定能够加速你代码执行的速度,更糟糕的是她有可能降低代码的执行速度。这取决于你的代码结构,当然很多情况下我们还是能够如愿以偿的。
如下:
/** * Gets the default toolkit. * * If a system property named "java.awt.headless" is set * to true then the headless implementation * of Toolkit is used. * * If there is no "java.awt.headless" or it is set to * false and there is a system property named * "awt.toolkit" , * that property is treated as the name of a class that is a subclass * of Toolkit ; * otherwise the default platform-specific implementation of * Toolkit is used. * * Also loads additional classes into the VM, using the property * 'assistive_technologies' specified in the Sun reference * implementation by a line in the 'accessibility.properties' * file. The form is "assistive_technologies=..." where * the "..." is a comma-separated list of assistive technology * classes to load. Each class is loaded in the order given * and a single instance of each is created using * Class.forName(class).newInstance(). This is done just after * the AWT toolkit is created. All errors are handled via an * AWTError exception. * @return the default toolkit. * @exception AWTError if a toolkit could not be found, or * if one could not be accessed or instantiated. */ public static synchronized Toolkit getDefaultToolkit() { if (toolkit == null) { try { // We disable the JIT during toolkit initialization. This
// tends to touch lots of classes that aren't needed again
// later and therefore JITing is counter-productiive.
java.lang.Compiler.disable(); java.security.AccessController.doPrivileged( new java.security.PrivilegedAction() { public Object run() { String nm = null; Class cls = null; try { nm = System.getProperty("awt.toolkit", "sun.awt.X11.XToolkit"); try { cls = Class.forName(nm); } catch (ClassNotFoundException e) { ClassLoader cl = ClassLoader.getSystemClassLoader(); if (cl != null) { try { cls = cl.loadClass(nm); } catch (ClassNotFoundException ee) { throw new AWTError("Toolkit not found: " + nm); } } } if (cls != null) { toolkit = (Toolkit)cls.newInstance(); if (GraphicsEnvironment.isHeadless()) { toolkit = new HeadlessToolkit(toolkit); } } } catch (InstantiationException e) { throw new AWTError("Could not instantiate Toolkit: " + nm); } catch (IllegalAccessException e) { throw new AWTError("Could not access Toolkit: " + nm); } return null; } }); loadAssistiveTechnologies(); } finally { // Make sure to always re-enable the JIT.
java.lang.Compiler.enable(); } } return toolkit; }
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从上面我们知道了之所以要关闭JITjava.lang.Compiler.disable(); 是因为加快执行的速度。由于JIT对每条字节码都进行编译,造成了编译过程负担过重。为了避免这种情况,当前的JIT只对经常执行的字节码进行编译,如循环等。
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