我看JAVA 之 Object & JNI
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：
-
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(); } } - 编译javac chapter01/TestJNI.java，生成TestJNI.class
-
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
-
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; } - 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 - 执行java程序 java -Djava.library.path=/Library/Java/JavaVirtualMachines/jdk-11.0.1.jdk/Contents/Home/include chapter01/TestJNI
