5.14 StringBuffer

1. 是什么

线程安全的、可变字符串 其实就是在StringBuilder的基础上加了synchronized关键字

2. 如何使用

public class TestStringBuilder
{
    public static void main(String[] args) throws InterruptedException
    {
        StringBuffer stringBuffer = new StringBuffer();

        Thread thread1 = new Thread(() -> {
            for (int i = 0; i < 5000; i++)
            {
                stringBuffer.append("aaaa");
            }
        });
        Thread thread2 = new Thread(() -> {
            for (int i = 0; i < 5000; i++)
            {
                stringBuffer.append("aaaa");
            }
        });

        thread1.start();
        thread2.start();

        thread1.join();
        thread2.join();


        System.out.println(stringBuffer.toString());
        System.out.println(stringBuffer.length() == 5000 * 2 * 4);//true

    }
}

3. 原理分析

3.1. 构造函数

 public final class StringBuffer//一样是final的
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{
	public StringBuffer() {
		//跟StringBuilder一样调用AbstractStringBuilder的构造方法
		super(16);//默认容量16个
	}

}

abstract class AbstractStringBuilder implements Appendable, CharSequence {

    char[] value;
    int count;

	AbstractStringBuilder(int capacity) {
		value = new char[capacity];
	}
}

3.2. append方法

//加了synchronized修饰
public synchronized StringBuffer append(String str) {
    toStringCache = null;
    super.append(str);
    return this;
}

3.3. toString

//加了synchronized修饰
public synchronized String toString() {
    if (toStringCache == null) {
        toStringCache = Arrays.copyOfRange(value, 0, count);
    }
    return new String(toStringCache, true);
}

3.4. subString

public synchronized String substring(int start, int end) {
	return super.substring(start, end);
}