Java6 String.substring()方法的内存泄露

substring(start,end)在Java编程里面常常使用，没想到若是使用不当，会出现内存泄露。

 

要了解substring()，最好的方法即是查看源码(jdk6)：

 1  /**
 2      * <blockquote><pre>
 3      * "hamburger".substring(4, 8) returns "urge"
 4      * "smiles".substring(1, 5) returns "mile"
 5      * </pre></blockquote>
 6      *
 7      * @param      beginIndex   the beginning index, inclusive.
 8      * @param      endIndex     the ending index, exclusive.
 9      * @return     the specified substring.
10      * @exception  IndexOutOfBoundsException  if the
11      *             <code>beginIndex</code> is negative, or
12      *             <code>endIndex</code> is larger than the length of
13      *             this <code>String</code> object, or
14      *             <code>beginIndex</code> is larger than
15      *             <code>endIndex</code>.
16      */
17     public String substring(int beginIndex, int endIndex) {
18     if (beginIndex < 0) {
19         throw new StringIndexOutOfBoundsException(beginIndex);
20     }
21     if (endIndex > count) {
22         throw new StringIndexOutOfBoundsException(endIndex);
23     }
24     if (beginIndex > endIndex) {
25         throw new StringIndexOutOfBoundsException(endIndex - beginIndex);
26     }
27     return ((beginIndex == 0) && (endIndex == count)) ? this :
28         new String(offset + beginIndex, endIndex - beginIndex, value);
29     }

 

插一句，这段substring()的源代码，为如何编写api提供了很好的一个例子，让我想起了老赵的一篇文章，对参数的判断，异常的处理，思路上有点接近。

值得注意的是，若是调用substring(i,i)的话（即beginIndex==endIndex）或者是substring(stringLength)（便是beginIndex==字符串长度），并不会抛出异常，而是会返回一个空的字符串，由于new String(offset + beginIndex , 0 , value)。

 

言归正传，真正建立字符串的，是一个String(int,in,char[])的构造函数，源代码以下：

1 // Package private constructor which shares value array for speed.
2     String(int offset, int count, char value[]) {
3     this.value = value;
4     this.offset = offset;
5     this.count = count;
6     }

 

Java里的字符串，实际上是由三个私有变量定义：

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence
{
    /** The value is used for character storage. */
    private final char value[];

    /** The offset is the first index of the storage that is used. */
    private final int offset;

    /** The count is the number of characters in the String. */
    private final int count;
}

 

当为字符串分配内存时，char数组存储字符，offset=0，count=字符串长度。问题在于，由substring(start,end)调用构造函数String(int,in,char[])时，其实是改变offset和count的位置达到取得子字符串的目的，而子字符串里的value[]数组，仍然指向原字符串。假设原字符串s有1GB，且咱们须要的是s.substring(1,10)这样一段小的字符串，但因为substring()里的value[]数组仍然指向1GB的原字符串，致使原字符串没法在GC中释放，从而产生了内存泄露。

 

但为何要这样设计呢？因为String是不可变的(immutable)，基于这种共享同一个字符数组的设计有如下好处：

调用substring()时无需复制数组，可重用value[]数组；且substring()的运行是常数时间，非线性，性能获得提升（这也是第二段代码注释的意思：share values for speed）。

而劣势，即是可能会产生内存泄露（实际上，Oracle早有人提出这个bug:http://bugs.sun.com/view_bug.do?bug_id=4513622）。

 

如何避免这个问题呢？有一个变通的方案，经过一个构造函数，复制一段数组：

 1 /**
 2      * Initializes a newly created {@code String} object so that it represents
 3      * the same sequence of characters as the argument; in other words, the
 4      * newly created string is a copy of the argument string. Unless an
 5      * explicit copy of {@code original} is needed, use of this constructor is
 6      * unnecessary since Strings are immutable.
 7      *
 8      * @param  original
 9      *         A {@code String}
10      */
11     public String(String original) {
12     int size = original.count;
13     char[] originalValue = original.value;
14     char[] v;
15       if (originalValue.length > size) {
16          // The array representing the String is bigger than the new
17          // String itself.  Perhaps this constructor is being called
18          // in order to trim the baggage, so make a copy of the array.
19             int off = original.offset;
20             v = Arrays.copyOfRange(originalValue, off, off+size);
21      } else {
22          // The array representing the String is the same
23          // size as the String, so no point in making a copy.
24         v = originalValue;
25      }
26     this.offset = 0;
27     this.count = size;
28     this.value = v;
29     }
30 
31 //smalStr no longer holds the value[] of 1GB
32 String smallStr = new String(s.substring(1,10));

 

上面的构造方法，从新复制了一段数组给v，而后再将v给字符串的数组，从而避免内存泄露。

 

在Java7里，String的实现已经改变，substring()方法的实现，由原来的共享数组变成了传统的拷贝，杜绝了内存泄露的同时也将运行时间由常数变成了线性：

 1 public String substring(int beginIndex, int endIndex) {
 2         if (beginIndex < 0) {
 3             throw new StringIndexOutOfBoundsException(beginIndex);
 4         }
 5         if (endIndex > value.length) {
 6             throw new StringIndexOutOfBoundsException(endIndex);
 7         }
 8         int subLen = endIndex - beginIndex;
 9         if (subLen < 0) {
10             throw new StringIndexOutOfBoundsException(subLen);
11         }
12         return ((beginIndex == 0) && (endIndex == value.length)) ? this
13                 : new String(value, beginIndex, subLen);
14     }

/**
     * Allocates a new {@code String} that contains characters from a subarray
     * of the character array argument. The {@code offset} argument is the
     * index of the first character of the subarray and the {@code count}
     * argument specifies the length of the subarray. The contents of the
     * subarray are copied; subsequent modification of the character array does
     * not affect the newly created string.
     *
     * @param  value
     *         Array that is the source of characters
     *
     * @param  offset
     *         The initial offset
     *
     * @param  count
     *         The length
     *
     * @throws  IndexOutOfBoundsException
     *          If the {@code offset} and {@code count} arguments index
     *          characters outside the bounds of the {@code value} array
     */
    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.value = Arrays.copyOfRange(value, offset, offset+count);
    }

 

这个构造函数，每次都会复制数组，实现与Java6并不同。至于哪一个好哪一个坏，其实很难说清楚。

听说有一种Rope的数据结构，能够更加高效地处理字符串，得好好看看。

 

参考：

http://javarevisited.blogspot.hk/2011/10/how-substring-in-java-works.html

http://eyalsch.wordpress.com/2009/10/27/stringleaks/

http://blog.zhaojie.me/2013/03/string-and-rope-1-string-in-dotnet-and-java.html

http://www.transylvania-jug.org/archives/5530