java源码分析-String类不可变性讨论

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {    
    /** The value is used for character storage. */
    private final char value[];    
    /** Cache the hash code for the string */
    private int hash; // Default to 0

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = -6849794470754667710L;    
    /**
     * Class String is special cased within the Serialization Stream Protocol.
     *
     * A String instance is written into an ObjectOutputStream according to
     * <a href="{@docRoot}/../platform/serialization/spec/output.html">
     * Object Serialization Specification, Section 6.2, "Stream Elements"</a>
     */
    private static final ObjectStreamField[] serialPersistentFields = new ObjectStreamField[0];

首先可以看到，String类使用了final修饰符，表明String类是不可继承的。
然后，我们主要关注String类的成员变量value,value是char[]类型，因为String对象实际上是用这个字符数组进行封装的。再看value的修饰符，使用了private，也没有提供setter方法，所以在String类的外部不能修改value，同时value也使用了final进行修饰，那么在String类的内部也不能修改value，但是上面final修饰引用类型变量的内容提到，这只能保证value不能指向其他的对象，但value指向的对象的状态是可以改变的。通过查看String类源码可以发现，String类不可变，关键是因为SUN公司的工程师，在后面所有String的方法里都很小心的没有去动字符数组里的元素。所以String类不可变的关键都在底层的实现，而不仅仅是一个final。

值得注意的是，hash没有用final修饰呢？我们来看hash的计算源码：

/**
     * Returns a hash code for this string. The hash code for a
     * {@code String} object is computed as
     * <blockquote><pre>
     * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
     * </pre></blockquote>
     * using {@code int} arithmetic, where {@code s[i]} is the
     * <i>i</i>th character of the string, {@code n} is the length of
     * the string, and {@code ^} indicates exponentiation.
     * (The hash value of the empty string is zero.)
     *
     * @return  a hash code value for this object.
     */
public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;
            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }

/**
 * 通过反射打破String的不可变性
 */
public class StringBreakImmutability {

    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        String str1 = new String("ABC");
        System.out.println("反射前："+str1);
        //获取Class对象
        Class clazz = str1.getClass();
        //获取字段Field对象信息
        Field value = clazz.getDeclaredField("value");
        value.setAccessible(true);
        //获取值
        char[] ch = (char[]) value.get(str1);
        ch[0] = 'B';
        System.out.println("反射后："+str1);
    }
}

public class JNIDemo {
	private String str = new String("hello java");
    {
        //系统加载其他语言的函数
        System.load("/home/sj/test/jni_string.so");
    }

    //native标识本地方法
    public native void stringJNI();

    public static void main(String[] args) {
       	JNIDemo demo = new JNIDemo();
        System.out.println("before jni, str:"+demo.str);   
        demo.stringJNI();
       	System.out.println("after jni, str:"+demo.str);   
    }
}

/usr/local/jdk1.8/bin/javac JNIDemo.java

/usr/local/jdk1.8/bin/javah JNIDemo

vim JNIDemo.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class JNIDemo */

#ifndef _Included_JNIDemo
#define _Included_JNIDemo
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     JNIDemo
 * Method:    stringJNI
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_JNIDemo_stringJNI
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

vim jni_string.c

#include <jni.h>
#include "JNIDemo.h"
#include <stdio.h>

JNIEXPORT void JNICALL Java_JNIDemo_stringJNI(JNIEnv *env,jobject obj){
	//得到java类名
    jclass java_class = (*env)->GetObjectClass(env,obj);
	//获取strin类型，注意string的类型签名有个分号；
    jfieldID id_str = (*env)->GetFieldID(env,java_class,"str","Ljava/lang/String;");
	//修改java的string成员变量值
    char* c_ch = "hello c";
	//字符数组c_ch转换成jstring类型   
    jstring cstr = (*env)->NewStringUTF(env,c_ch);
	//设置java的string类型变量s的值
    (*env)->SetObjectField(env,obj,id_str,cstr);
}

gcc  -fPIC -I /usr/local/jdk1.8/include  -I /usr/local/jdk1.8/include/linux   -shared -o jni_string.so jni_string.c