Java关于JIT的原理和相关知识

今天在读java.awt.Toolkit类时，遇到了JIT，代码如下：

/**
     * Gets the default toolkit.
     * <p>
     * If a system property named <code>"java.awt.headless"</code> is set
     * to <code>true</code> then the headless implementation
     * of <code>Toolkit</code> is used.
     * <p>
     * If there is no <code>"java.awt.headless"</code> or it is set to
     * <code>false</code> and there is a system property named
     * <code>"awt.toolkit"</code>,
     * that property is treated as the name of a class that is a subclass
     * of <code>Toolkit</code>;
     * otherwise the default platform-specific implementation of
     * <code>Toolkit</code> is used.
     * <p>
     * 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;
    }

1.JIT的工作原理图

工作原理
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。

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。

2.相关知识
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。

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。

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。