下面的一個例子說明String的substring方法引起的OutOfMemoryError問題：
[java] view plaincopyprint?public class TestGC {
private String large = new String(new char[100000]);
public String getSubString() {
return this.large.substring(0,2);
}
public static void main(String[] args) {
ArrayList<String> subStrings = new ArrayList<String>();
for (int i = 0; i <1000000; i++) {
TestGC testGC = new TestGC();
subStrings.add(testGC.getSubString());
}
}
}
public class TestGC {
private String large = new String(new char[100000]);
public String getSubString() {
return this.large.substring(0,2);
}
public static void main(String[] args) {
ArrayList<String> subStrings = new ArrayList<String>();
for (int i = 0; i <1000000; i++) {
TestGC testGC = new TestGC();
subStrings.add(testGC.getSubString());
}
}
}運行該程序，結果出現：
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
爲什麼會出現這個情況？查看一下JDK String類substring方法的源碼，可以找到原因，源碼如下：
[java] view plaincopyprint? public String substring(int beginIndex, int endIndex) {
if (beginIndex < 0) {
throw new StringIndexOutOfBoundsException(beginIndex);
}
if (endIndex > count) {
throw new StringIndexOutOfBoundsException(endIndex);
}
if (beginIndex > endIndex) {
throw new StringIndexOutOfBoundsException(endIndex - beginIndex);
}
return ((beginIndex == 0) && (endIndex == count)) ? this :
new String(offset + beginIndex, endIndex - beginIndex, value);
}
public String substring(int beginIndex, int endIndex) {
if (beginIndex < 0) {
throw new StringIndexOutOfBoundsException(beginIndex);
}
if (endIndex > count) {
throw new StringIndexOutOfBoundsException(endIndex);
}
if (beginIndex > endIndex) {
throw new StringIndexOutOfBoundsException(endIndex - beginIndex);
}
return ((beginIndex == 0) && (endIndex == count)) ? this :
new String(offset + beginIndex, endIndex - beginIndex, value);
}該方法最後一行，調用了String的一個私有的構造方法，如下：
[java] view plaincopyprint? // Package private constructor which shares value array for speed.
String(int offset, int count, char value[]) {
this.value = value;
this.offset = offset;
this.count = count;
}
// Package private constructor which shares value array for speed.
String(int offset, int count, char value[]) {
this.value = value;
this.offset = offset;
this.count = count;
}從該構造函數的訪問權限和註釋，可以看出，SUN爲了優化性能而專門寫了這個構造方法。
該方法爲了避免內存拷貝，提高性能，並沒有重新創建char數組，而是直接複用了原String對象的char[]，通過改變偏移
量和長度來標識不同的字符串內容。也就是說，substring出的來String小對象，仍然會指向原String大對象的char[]，
所以就導致了OutOfMemoryError問題。
找到問題之後，將上面代碼中，getSubString的方法修改一下，如下：
[java] view plaincopyprint?public String getSubString() {
return new String(this.large.substring(0,2));
}
public String getSubString() {
return new String(this.large.substring(0,2));
}將substring的結果，重新new一個String出來。再運行該程序，則沒有出現OutOfMemoryError的問題。爲什麼？因
爲此時調用的是String類的public的構造方法，該方法源碼如下：
[java] view plaincopyprint? public String(String original) {
int size = original.count;
char[] originalValue = original.value;
char[] v;
if (originalValue.length > size) {
// The array representing the String is bigger than the new
// String itself. Perhaps this constructor is being called
// in order to trim the baggage, so make a copy of the array.
int off = original.offset;
v = Arrays.copyOfRange(originalValue, off, off+size);
} else {
// The array representing the String is the same
// size as the String, so no point in making a copy.
v = originalValue;
}
this.offset = 0;
this.count = size;
this.value = v;
}
public String(String original) {
int size = original.count;
char[] originalValue = original.value;
char[] v;
if (originalValue.length > size) {
// The array representing the String is bigger than the new
// String itself. Perhaps this constructor is being called
// in order to trim the baggage, so make a copy of the array.
int off = original.offset;
v = Arrays.copyOfRange(originalValue, off, off+size);
} else {
// The array representing the String is the same
// size as the String, so no point in making a copy.
v = originalValue;
}
this.offset = 0;
this.count = size;
this.value = v;
}從代碼可以看出，在String對象中value的length大於count的情況下，會重新創建一個char[]，並進行內存拷貝。
除了substring方法之後，String的split方法，也存在同樣的問題，split的源碼如下：
[java] view plaincopyprint?public String[] split(String regex, int limit) {
urn Pattern.compile(regex).split(this, limit);
}
public String[] split(String regex, int limit) {
return Pattern.compile(regex).split(this, limit);
}可以看出，String的split方法通過Pattern的split方法來實現，Pattern的split方法源碼如下：
[java] view plaincopyprint?public String[] split(CharSequence input, int limit) {
int index = 0;
boolean matchLimited = limit > 0;
ArrayList<String> matchList = new ArrayList<String>();
Matcher m = matcher(input);
// Add segments before each match found
while(m.find()) {
if (!matchLimited || matchList.size() < limit - 1) {
String match = input.subSequence(index, m.start()).toString();
matchList.add(match);
index = m.end();
} else if (matchList.size() == limit - 1) { // last one
String match = input.subSequence(index,
input.length()).toString();
matchList.add(match);
index = m.end();
}
}
// If no match was found, return this
if (index == 0)
return new String[] {input.toString()};
// Add remaining segment
if (!matchLimited || matchList.size() < limit)
matchList.add(input.subSequence(index, input.length()).toString());
// Construct result
int resultSize = matchList.size();
if (limit == 0)
while (resultSize > 0 && matchList.get(resultSize-1).equals(""))
resultSize--;
String[] result = new String[resultSize];
return matchList.subList(0, resultSize).toArray(result);
}
public String[] split(CharSequence input, int limit) {
int index = 0;
boolean matchLimited = limit > 0;
ArrayList<String> matchList = new ArrayList<String>();
Matcher m = matcher(input);
// Add segments before each match found
while(m.find()) {
if (!matchLimited || matchList.size() < limit - 1) {
String match = input.subSequence(index, m.start()).toString();
matchList.add(match);
index = m.end();
} else if (matchList.size() == limit - 1) { // last one
String match = input.subSequence(index,
input.length()).toString();
matchList.add(match);
index = m.end();
}
}
// If no match was found, return this
if (index == 0)
return new String[] {input.toString()};
// Add remaining segment
if (!matchLimited || matchList.size() < limit)
matchList.add(input.subSequence(index, input.length()).toString());
// Construct result
int resultSize = matchList.size();
if (limit == 0)
while (resultSize > 0 && matchList.get(resultSize-1).equals(""))
resultSize--;
String[] result = new String[resultSize];
return matchList.subList(0, resultSize).toArray(result);
}方法中的第9行：Stirng match = input.subSequence(intdex, m.start()).toString();
調用了String類的subSequence方法，該方法源碼如下：
[java] view plaincopyprint?public CharSequence subSequence(int beginIndex, int endIndex) {
return this.substring(beginIndex, endIndex);
}
public CharSequence subSequence(int beginIndex, int endIndex) {
return this.substring(beginIndex, endIndex);
}通過代碼可以看出，最終調用的是String類的substring方法，因此存在同樣的問題。split出來的小對象，直接使
用原String對象的char[]。
看了一下StringBuilder和StringBuffer的substring方法，則不存在這樣的問題。其源碼如下：
[java] view plaincopyprint?public String substring(int start, int end) {
(start < 0)
throw new StringIndexOutOfBoundsException(start);
(end > count)
throw new StringIndexOutOfBoundsException(end);
(start > end)
throw new StringIndexOutOfBoundsException(end - start);
return new String(value, start, end - start);
}
public String substring(int start, int end) {
if (start < 0)
throw new StringIndexOutOfBoundsException(start);
if (end > count)
throw new StringIndexOutOfBoundsException(end);
if (start > end)
throw new StringIndexOutOfBoundsException(end - start);
return new String(value, start, end - start);
}最後一行，調用了String類的public構造方法，方法源碼如下：
[java] view plaincopyprint?public String(char value[], int offset, int count) {
if (offset < 0) {
throw new StringIndexOutOfBoundsException(offset);
}
if (count < 0) {
throw new StringIndexOutOfBoundsException(count);
}
// Note: offset or count might be near -1>>>1.
if (offset > value.length - count) {
throw new StringIndexOutOfBoundsException(offset + count);
}
this.offset = 0;
this.count = count;
this.value = Arrays.copyOfRange(value, offset, offset+count);
}
public String(char value[], int offset, int count) {
if (offset < 0) {
throw new StringIndexOutOfBoundsException(offset);
}
if (count < 0) {
throw new StringIndexOutOfBoundsException(count);
}
// Note: offset or count might be near -1>>>1.
if (offset > value.length - count) {
throw new StringIndexOutOfBoundsException(offset + count);
}
this.offset = 0;
this.count = count;
this.value = Arrays.copyOfRange(value, offset, offset+count);
}該方法不是直接使用原String對象的char[]，而是重新進行了內存拷貝。
