StringBuilder 与 StringBuffer

可变性

• String 不可变

在 String 中， byte[] value 由 final 修饰

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence,
               Constable, ConstantDesc {

    /**
     * The value is used for character storage.
     *
     * @implNote This field is trusted by the VM, and is a subject to
     * constant folding if String instance is constant. Overwriting this
     * field after construction will cause problems.
     *
     * Additionally, it is marked with {@link Stable} to trust the contents
     * of the array. No other facility in JDK provides this functionality (yet).
     * {@link Stable} is safe here, because value is never null.
     */
    @Stable
    private final byte[] value;

    ...
}

• StringBuffer 和 StringBuilder 可变

StringBuffer 和 StringBulder 都继承于 AbstractStringBuilder 抽象类，其中的 byte[] value 没有修饰符

abstract sealed class AbstractStringBuilder implements Appendable, CharSequence
    permits StringBuilder, StringBuffer {
    /**
     * The value is used for character storage.
     */
    byte[] value;

    ...
}

线程安全

• String 不可变，因此是线程安全的

• StringBuilder 不是线程安全的

以 append() 为例，其方法没有修饰符

public final class StringBuilder
    extends AbstractStringBuilder
    implements Appendable, java.io.Serializable, Comparable<StringBuilder>, CharSequence
{
    ...

    public StringBuilder append(StringBuffer sb) {
        super.append(sb);
        return this;
    }

    ...
}

• StringBuffer 是线程安全的，内部使用 synchronized 进行同步

以 append() 为例，其方法被 synchronized 修饰

public final class StringBuffer
    extends AbstractStringBuilder
    implements Appendable, Serializable, Comparable<StringBuffer>, CharSequence
{
    ...

    @Override
    synchronized StringBuffer append(AbstractStringBuilder asb) {
        toStringCache = null;
        super.append(asb);
        return this;
    }

    ...
}