手把手实例对比String、StringBuilder字符串的连接效率及StringBuilder和StringBuffer线程安全的比较...

private void ensureCapacityInternal(int minimumCapacity) {
         // 最小所需容量minimumCapacity是否比原数组长度要长
        // overflow-conscious code
        if (minimumCapacity - value.length > 0) {
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }
private int newCapacity(int minCapacity) {
         // 计算扩容之后的容量newCapacity
        // overflow-conscious code
        int newCapacity = (value.length << 1) + 2;
        // 扩容后还小于所需的最小容量
        if (newCapacity - minCapacity < 0) {
            // 设置新容量为最小所需容量minimumCapacity
            newCapacity = minCapacity;
        }
        // newCapacity是否溢出，newCapacity是否比数组所能分配的最大容量 MAX_ARRAY_SIZE 还要大。
        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
    }
    private int hugeCapacity(int minCapacity) {
        // 最小所需容量minCapacity大于Integer.MAX_VALUE时抛出内存溢出异常
        if (Integer.MAX_VALUE - minCapacity < 0) { // overflow
            throw new OutOfMemoryError();
        }
        // 如果minCapacity介于MAX_ARRAY_SIZE和Integer.MAX_VALUE之间，则新的容量为minCapacity，否则直接使用MAX_ARRAY_SIZE作为新的容量。
        return (minCapacity > MAX_ARRAY_SIZE)
            ? minCapacity : MAX_ARRAY_SIZE;
    }

/**
 * 比较字符串连接速度
 *
 * @Author: lingyejun
 * @Date: 2019/8/17
 * @Describe:
 * @Modified By:
 */
public class LinkCompare {
    /**
     * 原始字符串连接
     *
     * @param times
     */
    public static void linkByString(int times) {
        Long startTime = System.currentTimeMillis();
        String initStr = "";
        for (int i = 0; i < times; i++) {
            initStr = initStr + i;
        }
        Long endTime = System.currentTimeMillis();
        System.out.println("String 连接 " + times + " 次 消耗:" + (endTime - startTime) + "ms");
    }
    /**
     * 使用StringBuilder连接字符串
     *
     * @param times
     */
    public static void linkByStringBuilder(int times) {
        Long startTime = System.currentTimeMillis();
        StringBuilder initStr = new StringBuilder();
        for (int i = 0; i < times; i++) {
            initStr.append(i);
        }
        Long endTime = System.currentTimeMillis();
        System.out.println("StringBuilder 连接 " + times + " 次 消耗:" + (endTime - startTime) + "ms");
    }
    /**
     * 使用StringBuffer连接字符串
     *
     * @param times
     */
    public static void linkByStringBuffer(int times) {
        Long startTime = System.currentTimeMillis();
        StringBuffer initStr = new StringBuffer();
        for (int i = 0; i < times; i++) {
            initStr.append(i);
        }
        Long endTime = System.currentTimeMillis();
        System.out.println("StringBuffer 连接 " + times + " 次 消耗:" + (endTime - startTime) + "ms");
    }
    public static void main(String[] args) {
        // 100000000
        linkByStringBuilder(40000);
        //-XX:+PrintGCDetails
        //linkByString(40000);
    }
}

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

import java.util.ArrayList;
import java.util.List;
/**
 * StringBuilder和StringBuffer的并发测验
 *
 * @Author: lingyejun
 * @Date: 2019/8/17
 * @Describe:
 * @Modified By:
 */
public class SecurityCompare {
    public void stringBuilderTest() {
        // 初始化StringBuilder
        StringBuilder stringBuilder = new StringBuilder();
        // joinList
        List<StringBuilderThread> joinList = new ArrayList<>();
        // 模拟并发场景
        for (int i = 0; i < 1000; i++) {
            StringBuilderThread sbt = new StringBuilderThread(stringBuilder);
            sbt.start();
            joinList.add(sbt);
        }
        // 等待append线程执行完毕后再执行主线程
        for (StringBuilderThread thread : joinList) {
            try {
                thread.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        // 打印最终的结果
        System.out.println("StringBuilder 并发append的结果： " + stringBuilder.length());
    }
    public void stringBufferTest() {
        // 初始化StringBuffer
        StringBuffer stringBuffer = new StringBuffer();
        // joinList
        List<StringBufferThread> joinList = new ArrayList<>();
        // 模拟并发场景
        for (int i = 0; i < 1000; i++) {
            StringBufferThread sbf = new StringBufferThread(stringBuffer);
            sbf.start();
            joinList.add(sbf);
        }
        // 等待append线程执行完毕后再执行主线程
        for (StringBufferThread thread : joinList) {
            try {
                thread.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        // 打印最终的结果
        System.out.println("StringBuffer 并发append的结果： " + stringBuffer.length());
    }
    public static void main(String[] args) {
        SecurityCompare securityCompare = new SecurityCompare();
        securityCompare.stringBuilderTest();
        securityCompare.stringBufferTest();
    }
    public static class StringBuilderThread extends Thread {
        private StringBuilder stringBuilder;
        public StringBuilderThread(StringBuilder stringBuilder) {
            this.stringBuilder = stringBuilder;
        }
        @Override
        public void run() {
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            stringBuilder.append("a");
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    private static class StringBufferThread extends Thread {
        private StringBuffer stringBuffer;
        public StringBufferThread(StringBuffer stringBuffer) {
            this.stringBuffer = stringBuffer;
        }
        @Override
        public void run() {
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            stringBuffer.append("a");
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}