我看JAVA 之 Object & JNI

注：基于jdk11

Object

Object类是java语言中所有类的父类。

 public class Object { //注册本地函数，方可以从库中的本机代码调用JNI函数 private static native void registerNatives(); static { registerNatives(); } /** * Constructs a new object. */ //HotSpotIntrinsicCandidate注解，说明本方法，jvm额外维护了一个利用了CPU指令的高效实现 @HotSpotIntrinsicCandidate public Object() {} //返回对象的运行时类型 /** * Number c = 0; * Class<? extends Number> clazz = c.getClass(); * System.out.println(clazz); * 打印如下： * class java.lang.Integer * 关于泛型及泛型擦除概念，见后续泛型章节 */ @HotSpotIntrinsicCandidate public final native Class<?> getClass(); /** * 返回对象的hash码，hashCode方法支持hash表（比如java.util.HashMap）的特性， * 不同的对象返回唯一的has码，可以提高hash表数据结构的性能。 * hashCode有时候以对象内存地址的机制生成，有时候不是 * java.lang.System.identityHashCode 工具方法 返回默认hashCode()方法的值，null引用返回的hash码为0 */ @HotSpotIntrinsicCandidate public native int hashCode(); /** * 标示其他对象与当前对象是否相等 * 自反性: 自己（非null）与自己比较永远相当 * 对称性: x vs y or y vs x 等价 * 可传递: x.equals(y) == true and y.equals(z), then x.equals(z) * 一致性: 一旦x.equals(y) == true, then anytime x.equals(y) == true * any not null value not equals null * 如果覆盖equals方法，一定要覆盖hashCode方法 */ public boolean equals(Object obj) { return (this == obj); } /** * clone返回会返回一个相同类型的新对象拷贝，如果要实现克隆，那么当前类及其父类需要实现cloneable接口。 * 对于复杂类型，默认的clone方式仅实现”浅克隆”，如果要实现”深克隆”，需要覆盖clone方法。 * 注：在软件编程中，推荐使用工具类的方式做copy，而不是覆盖clone的方式 */ @HotSpotIntrinsicCandidate protected native Object clone() throws CloneNotSupportedException; /** * 默认返回 getClass().getName() + '@' + Integer.toHexString(hashCode()) * 具体类可以覆盖Object的toString()方法 */ public String toString() { return getClass().getName() + "@" + Integer.toHexString(hashCode()); } /** * 持有该对象锁的线程通过调用notify唤醒等待此对象监视器的某一个线程，选择是随机的（具体由不同虚拟机实现）。一次只能有一个线程持有对象锁。 * 获取对象锁定三种方式如下： * 1，通过执行该对象的同步实例方法 * 2，通过执行该对象的同步代码块 * 3，通过执行类的同步静态方法 * 见 /openjdk/hotspot/src/share/vm/prims/jvm.cpp line: 526~530 * 见 /openjdk/hotspot/src/share/vm/runtime/synchronizer.cpp line: 407~411 * @throws IllegalMonitorStateException if the current thread is not * the owner of this object's monitor. * @see java.lang.Object#notifyAll() * @see java.lang.Object#wait() */ @HotSpotIntrinsicCandidate public final native void notify(); /** * 持有该对象锁的线程通过调用notify唤醒等待此对象监视器的所有线程。当当前线程释放对象监视器后，其他唤醒的线程会竞争对象监视器。 * * @throws IllegalMonitorStateException if the current thread is not * the owner of this object's monitor. * @see java.lang.Object#notify() * @see java.lang.Object#wait() */ @HotSpotIntrinsicCandidate public final native void notifyAll(); /** * 调用此方法会导致当前对象进入等待状态，知道被其他线程唤醒（notify）或中断（InterruptedException）。相当于wait(0L) * * @throws IllegalMonitorStateException if the current thread is not * the owner of the object's monitor * @throws InterruptedException if any thread interrupted the current thread before or * while the current thread was waiting. The <em>interrupted status</em> of the * current thread is cleared when this exception is thrown. * @see #notify() * @see #notifyAll() * @see #wait(long) * @see #wait(long, int) */ public final void wait() throws InterruptedException { wait(0L); } /** * * @param timeoutMillis the maximum time to wait, in milliseconds * @throws IllegalArgumentException if {@code timeoutMillis} is negative * @throws IllegalMonitorStateException if the current thread is not * the owner of the object's monitor * @throws InterruptedException if any thread interrupted the current thread before or * while the current thread was waiting. The <em>interrupted status</em> of the * current thread is cleared when this exception is thrown. * @see #notify() * @see #notifyAll() * @see #wait() * @see #wait(long, int) */ public final native void wait(long timeoutMillis) throws InterruptedException; /** * 在并发编程章节重点讲解 * @param timeoutMillis the maximum time to wait, in milliseconds * @param nanos additional time, in nanoseconds, in the range range 0-999999 inclusive * @throws IllegalArgumentException if {@code timeoutMillis} is negative, * or if the value of {@code nanos} is out of range * @throws IllegalMonitorStateException if the current thread is not * the owner of the object's monitor * @throws InterruptedException if any thread interrupted the current thread before or * while the current thread was waiting. The <em>interrupted status</em> of the * current thread is cleared when this exception is thrown. * @see #notify() * @see #notifyAll() * @see #wait() * @see #wait(long) */ public final void wait(long timeoutMillis, int nanos) throws InterruptedException { if (timeoutMillis < 0) { throw new IllegalArgumentException("timeoutMillis value is negative"); } if (nanos < 0 || nanos > 999999) { throw new IllegalArgumentException( "nanosecond timeout value out of range"); } if (nanos > 0) { timeoutMillis++; } wait(timeoutMillis); } /** * 通知垃圾回收器当前对应已经没有引用了，具体回收还要看jvm内部实现。 * jdk9 标记位过时，在jvm原理章节重点讲解 * * @throws Throwable the {@code Exception} raised by this method * @see java.lang.ref.WeakReference * @see java.lang.ref.PhantomReference * @jls 12.6 Finalization of Class Instances */ @Deprecated(since="9") protected void finalize() throws Throwable { } } 

JNI(Java Native Interface)

java本地接口，意为JAVA语言提供一套规范供操作系统底层实现，一般是C/C++语言，不同操作系统版本的jvm提供了相应的实现，以达到"Write Once Run Anywhere“

实现JNI大致步骤如下：

1. 定义java程序

   package chapter01; public class TestJNI { public native void hello();//所有native关键词修饰的都是对本地的声明 static { System.loadLibrary("hello");//载入本地库 } public static void main(String[] args) { new TestJNI().hello(); } }

2. 编译javac chapter01/TestJNI.java，生成TestJNI.class

3. 执行javac chapter01/TestJNI.java -h . ，生成JAVA本地接口chapter01_TestJNI.h

   /* DO NOT EDIT THIS FILE - it is machine generated */ #include <jni.h> /* Header for class chapter01_TestJNI */ #ifndef _Included_chapter01_TestJNI #define _Included_chapter01_TestJNI #ifdef __cplusplus extern "C" { #endif /* * Class: chapter01_TestJNI * Method: hello * Signature: ()V */ JNIEXPORT void JNICALL Java_chapter01_TestJNI_hello (JNIEnv *, jobject); #ifdef __cplusplus } #endif #endif

   注意：JNIEnv、 jobject等类型都是在jni.h头文件中定义的，所以需要include jni.h

4. 编写步骤3的接口实现，创建TestJNIImpl.c

   #include "jni.h" #include "chapter01_TestJNI.h" //#include otherheaders JNIEXPORT void JNICALL Java_chapter01_TestJNI_hello(JNIEnv *env, jobject obj) { printf("Helloworld!\n"); return; }

5. 将本地方法编写的文件生成动态链接库
   gcc -dynamiclib -I /Library/Java/JavaVirtualMachines/jdk-11.0.1.jdk/Contents/Home/include TestJNIImpl.c -o libhello.jnilib
   注意：如果/Library/Java/JavaVirtualMachines/jdk-11.0.1.jdk/Contents/Home/include不存在jni_md.h的话，可以从 /Library/Java/JavaVirtualMachines/jdk-11.0.1.jdk/Contents/Home/include/darwin 拷贝过来
6. 执行java程序 java -Djava.library.path=/Library/Java/JavaVirtualMachines/jdk-11.0.1.jdk/Contents/Home/include chapter01/TestJNI