创建一个可变的java.lang.String [英] Create a mutable java.lang.String

问题描述

众所周知，Java String 是不可变的。从一开始，不可变字符串就是java的重要补充。与C风格的字符串相比，不变性允许快速访问和大量优化，显着减少了错误，并有助于强制执行安全模型。

It's common knowledge that Java Strings are immutable. Immutable Strings are great addition to java since its inception. Immutability allows fast access and a lot of optimizations, significantly less error-prone compared to C-style strings, and helps enforce the security model.

可以创建一个可变的一个没有使用黑客攻击，即

It's possible to create a mutable one without using hacks, namely

• java.lang.reflect


• sun.misc.Unsafe


• bootstrap类加载器中的类


• JNI（或JNA，因为它需要JNI）

• java.lang.reflect
• sun.misc.Unsafe
• Classes in bootstrap classloader
• JNI (or JNA as it requires JNI)

但它是否可能只是普通的Java，所以字符串可以修改为随时？问题是如何？

But is it possible in just plain Java, so that the string can be modified at any time? The question is How?

推荐答案

创建 java.lang.String 使用Charset构造函数，可以注入自己的Charset，它带来你自己的 CharsetDecoder 。 CharsetDecoder 在decodeLoop方法中获取对 CharBuffer 对象的引用。 CharBuffer包装原始String对象的char []。由于CharsetDecoder具有对它的引用，您可以使用CharBuffer更改底层char []，因此您有一个可变的String。

Creating a java.lang.String with the Charset constructor, one can inject your own Charset, which brings your own CharsetDecoder. The CharsetDecoder gets a reference to a CharBuffer object in the decodeLoop method. The CharBuffer wraps the char[] of the original String object. Since the CharsetDecoder has a reference to it, you can change the underlying char[] using the CharBuffer, thus you have a mutable String.

public class MutableStringTest {


    // http://stackoverflow.com/questions/11146255/how-to-create-mutable-java-lang-string#11146288
    @Test
    public void testMutableString() throws Exception {
        final String s = createModifiableString();
        System.out.println(s);
        modify(s);
        System.out.println(s);
    }

    private final AtomicReference<CharBuffer> cbRef = new AtomicReference<CharBuffer>();
    private String createModifiableString() {
        Charset charset = new Charset("foo", null) {
            @Override
            public boolean contains(Charset cs) {
                return false;
            }

            @Override
            public CharsetDecoder newDecoder() {
                CharsetDecoder cd = new CharsetDecoder(this, 1.0f, 1.0f) {
                    @Override
                    protected CoderResult decodeLoop(ByteBuffer in, CharBuffer out) {
                        cbRef.set(out);
                        while(in.remaining()>0) {
                            out.append((char)in.get());
                        }
                        return CoderResult.UNDERFLOW;
                    }
                };
                return cd;
            }

            @Override
            public CharsetEncoder newEncoder() {
                return null;
            }
        };
        return new String("abc".getBytes(), charset);
    }
    private void modify(String s) {
        CharBuffer charBuffer = cbRef.get();
        charBuffer.position(0);
        charBuffer.put("xyz");
    }

}

运行代码打印

abc
zzz

我不知道如何正确实现decodeLoop（），但我现在不在乎：）

I don't know how to correctly implement decodeLoop(), but i don't care right now :)

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