StringBuilder、StringBuffer源碼分析

StringBuilder源碼分析

類結構

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence

StringBuilder使用final關鍵字修飾，和String同樣不能夠被繼承java

StringBuilder繼承AbstractStringBuilder並實現了Serializable和CharSequence，能夠被序列化數組

方法

StringBuilder 的方法可能是直接調用父類AbstractStringBuilder的方法，這裏找幾個典型的方法看一下安全

StringBuilder append(Object obj)方法重寫父類的方法，追加Object類型的元素

@Override
    public StringBuilder append(Object obj) {
        return append(String.valueOf(obj));//String.valueOf(obj)獲取對象轉換成的字符串
    }
public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
    }
@Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }
public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();//若是爲null追加字符串「null」
        int len = str.length();
        ensureCapacityInternal(count + len);
  //拷貝字符串到數組
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

StringBuilder delete(int start, int end)刪除指定起點下標到指定結束下標的字符

@Override
    public StringBuilder delete(int start, int end) {
        super.delete(start, end);
        return this;
    }
public AbstractStringBuilder delete(int start, int end) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (end > count)//若是結束下標>當前保存char的最大下標，直接賦值爲最大下標
            end = count;
        if (start > end)
            throw new StringIndexOutOfBoundsException();
        int len = end - start;
        if (len > 0) {
          //把刪除尾下標後的元素拷貝到刪除起始下標後
            System.arraycopy(value, start+len, value, start, count-end);
            count -= len;
        }
        return this;
    }

StringBuilder replace(int start, int end, String str)使用字符串替換指定範圍內的字符

@Override
    public StringBuilder replace(int start, int end, String str) {
        super.replace(start, end, str);
        return this;
    }
 public AbstractStringBuilder replace(int start, int end, String str) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (start > count)
            throw new StringIndexOutOfBoundsException("start > length()");
        if (start > end)
            throw new StringIndexOutOfBoundsException("start > end");

        if (end > count)
            end = count;
        int len = str.length();
   //計算須要的容量
        int newCount = count + len - (end - start);
   //擴容
        ensureCapacityInternal(newCount);
	//刪除指定範圍的字符
        System.arraycopy(value, end, value, start + len, count - end);
   //在刪除的起始位置插入字符串
        str.getChars(value, start);
        count = newCount;
        return this;
    }

StringBuilder insert(int offset, Object obj)在指定位置插入對象

@Override
    public StringBuilder insert(int offset, Object obj) {
            super.insert(offset, obj);
            return this;
    }
public AbstractStringBuilder insert(int offset, Object obj) {
        return insert(offset, String.valueOf(obj));//String.valueOf(obj)獲取對象轉換的字符串
    }
public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
    }
 @Override
    public StringBuilder insert(int offset, String str) {
        super.insert(offset, str);
        return this;
    }
public AbstractStringBuilder insert(int offset, String str) {
        if ((offset < 0) || (offset > length()))
            throw new StringIndexOutOfBoundsException(offset);
        if (str == null)
            str = "null";
        int len = str.length();
  	//擴容
        ensureCapacityInternal(count + len);
  //把要插入位置後必定數量的字符（插入字符串長度）串移動後移必定距離（插入字符串長度）
        System.arraycopy(value, offset, value, offset + len, count - offset);
  	//插入要插入的字符串
        str.getChars(value, offset);
        count += len;
        return this;
    }

能夠看到，StringBuilder的append、insert、replace、delete都是對父類的char數組進行的一些操做，並無產生新的對象app

String toString() 最精髓的一個方法

@Override
    public String toString() {
        //把進過一些列修改後的最終char數組生成String
        return new String(value, 0, count);
    }

這裏咱們看到在toString的時候，把char數組生成了String，這也是爲何StringBuilder比String效率高的緣由，String類沒作一點修改都會生成新的對象，那麼在頻繁拼串和截取字符串時，效率固然不如StringBuilderide

StringBuffer源碼分析

類結構

public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence

StringBuffer的類結構和StringBuilder的同樣源碼分析

方法

StringBuffer和StringBuilder同樣，不少方法都是調用父類AbstractStringBuilder的方法,咱們看幾個最主要的方法ui

StringBuffer append(Object obj)向StringBuffer中追加對象,和StringBuilder的追加對象同樣的代碼

@Override
    public synchronized StringBuffer append(Object obj) {
        toStringCache = null;
        super.append(String.valueOf(obj));
        return this;
    }
public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }
public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {
        if (srcBegin < 0) {
            throw new StringIndexOutOfBoundsException(srcBegin);
        }
        if (srcEnd > value.length) {
            throw new StringIndexOutOfBoundsException(srcEnd);
        }
        if (srcBegin > srcEnd) {
            throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);
        }
        System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
    }

StringBuffer delete(int start, int end)刪除指定範圍內的字符,和StringBuilder中delete方法代碼同樣

@Override
    public synchronized StringBuffer delete(int start, int end) {
        toStringCache = null;
        super.delete(start, end);
        return this;
    }
public AbstractStringBuilder delete(int start, int end) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (end > count)
            end = count;
        if (start > end)
            throw new StringIndexOutOfBoundsException();
        int len = end - start;
        if (len > 0) {
            System.arraycopy(value, start+len, value, start, count-end);
            count -= len;
        }
        return this;
    }

StringBuffer replace(int start, int end, String str)方法使用字符串替換指定範圍內的字符,和StringBuilder的replace方法代碼同樣

@Override
    public synchronized StringBuffer replace(int start, int end, String str) {
        toStringCache = null;
        super.replace(start, end, str);
        return this;
    }
public AbstractStringBuilder replace(int start, int end, String str) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (start > count)
            throw new StringIndexOutOfBoundsException("start > length()");
        if (start > end)
            throw new StringIndexOutOfBoundsException("start > end");

        if (end > count)
            end = count;
        int len = str.length();
        int newCount = count + len - (end - start);
        ensureCapacityInternal(newCount);

        System.arraycopy(value, end, value, start + len, count - end);
        str.getChars(value, start);
        count = newCount;
        return this;
    }

StringBuffer insert(int offset, Object obj)在指定位置插入字符串，也是和StringBuilder的insert方法代碼同樣

@Override
    public synchronized StringBuffer insert(int offset, Object obj) {
        toStringCache = null;
        super.insert(offset, String.valueOf(obj));
        return this;
    }
public AbstractStringBuilder insert(int offset, String str) {
        if ((offset < 0) || (offset > length()))
            throw new StringIndexOutOfBoundsException(offset);
        if (str == null)
            str = "null";
        int len = str.length();
        ensureCapacityInternal(count + len);
        System.arraycopy(value, offset, value, offset + len, count - offset);
        str.getChars(value, offset);
        count += len;
        return this;
    }

經過分析這幾個方法源碼，咱們能夠看到，StringBuilder和StringBuffer在方法的實現上是一致的，惟一的區別是StringBuffer的全部方法都加了synchronized鎖，因此是線程安全的this

String toString()把StringBuffer轉換成字符串

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

StringBuffer與StringBuilder都是在修改的時候並無產生新的對象，只是在調用toString方法是才轉換爲字符串。spa

總結

StringBuilder和StringBuffer的類結構是一致的，都是使用父類的char數組保存字符。
StringBuffer的全部方法都加了synchronized鎖，因此是線程安全的，可是這也使得它的效率比StringBuilder低。
StringBuilder和StringBuffer的基本思想是一致的，對StringBuilder、StringBuffer的任何修改都不會產生新對象，這也使得StringBuilder、StringBuffer在進行大量拼串截取時比String的效率高。

StringBuilder、StringBuffer分析比較